During an episode of Magical Project S, Misao dreams about having magic powers and the "fun" she would have using them. When she actually got said powers, she becomes the arguably insane persona of Pixy Misa (who tortures the entire main cast).
In Haruhi Suzumiya, the titular character is kept Locked Out of the Loop regarding the fact that her friends are all examples of the very weirdness she seeks to find for this very reason. Already a Jerk AssTsundere, they're afraid that if she discovers that aliens, espers, dimension/time travellers and other such entities are real, she will manage to make the intuitive leap and realise that she is a Reality Warper of such power that she is, in all practical terms, a goddess. Given how much strain she can put on the fabric of reality even while she's unaware of her power, they naturally fear that allowing someone of her attitudes and ethics full control of her abilities would effectively bump her to Eldritch Abomination status.
It's implied that this has already happened a number of times, and the current reality is just one of many she has created. The current one was created three years ago, but because she created the reality as though it had always existed, only aliens, time travellers and espers with the ability to sense Haruhi's intense power realised what happened.
Cowboy Bebop, "Pierrot le Fou": A government test subject is given superhuman assassin skills, and then goes crazy as a side-effect. Given the methods shown (the fact that no explanation of any sort is given for what, precisely, is being done, it's all the more disturbing), it's not too surprising.
In The Big O, Straw Nihilist villain Schwarzwald is the only known character to figure out the big secret. As a result, he goes batshit insane, wraps himself up like a mummy, and spends the rest of the series raving about philosophy, leaving typewriters lying around everywhere he goes, and showing up in a robot several weeks after his death to chew out another villain who gets killed trying to use it.
Mobile Suit Gundam: A common theme in the franchise, most notably Zeta Gundam and Gundam ZZ where all Newtypes are often mentally unstable in some fashion, may have forms of amnesia, be brainwashed, though in some other cases they're just plain stubborn. This is particularly true of "Cyber-Newtypes", people who originally had no special powers but had their brains surgically altered (almost always against their will) to become Newtypes.
Gundam SEED plays on this by giving performance-enhancing drugs and treatments to criminals who are doing it in exchange for a full pardon. Naturally, this drives them to insanity in combat... which is what was planned. As a result, they are given drugs in such doses that by the time combat is over, they are having withdrawal symptoms and are manageable again. Withdrawal will also kill them if they are deprived of the drugs for too long, thus ensuring their loyalty.
In Gundam Wing, the ZERO System gives the person who uses it incredible reaction times and tactical predictions bordering on prescience. The problem is, the System evaluates everything as a potential threat, and plugs into the user's mind all of the possible attacks that could happen. If he can't focus on the battle, those violent predictions start afflicting whatever he starts thinking about (like, say, his girlfriend, or that nice peaceful space colony over there), and soon enough he's a psychopath slaughtering whatever the System says is his enemy.
Similarly, Gasaraki has mecha pilots who were given a cocktail of boosting drugs in order to improve their battle performance (without their knowledge or consent, and said drugs was actually fluid extracted from the muscles of a 1000 year old demon), and the inevitably go berserk from the effects, before either lapsing into a coma or suffering cardiac arrest.
Anime classic AKIRA centers around the results of government experimentation on a Japanese gang member with a serious inferiority complex. As a result, the newly created Big Bad Tetsuo runs amok with his telekinetic powers until he mutates, explodes, and forms either a new universe or... something.
Death Note drops some hints that Light Yagami wasn't entirely stable before gaining the power to kill anyone in the world at any time, but his sanity certainly heads downwards after that. In fact, all the Kiras in the series seem to go bonkers after picking up a Death Note, since it seems to take the major flaw in their personality and magnify it — Light and Takada's narcissism, Mikami's fanatical desire for justice, Misa's obsessiveness, and Higuchi's greed.
It even says in chapter 19 that "with great power comes great evil," which is pretty darn close, in Light's case.
When he briefly lost his memories of the Death Note as part of a Memory Gambit, he immediately becomes a Nice Guy that has almost nothing in common with "Kira", to the point that he finds the very notion that he might have been "Kira" to be horrifying. The shock of his first kill and the resulting god complex he manifested to avoid the guilt really messed him up.
The Dragon Ball Z movies have Broly, who's more or less the personification of this trope (although it is also heavily implied that the life-threatening experiences of his childhood also contributed quite a bit to his insanity as well).
It's all but stated in the manga that reaching new levels of Super Saiyan cause Goku and Gohan to become more aggressive and ruthless, which is particularly jarring to the other characters (Vegeta was already pretty bad, so he just gets more and more arrogant). This probably isn't helped because Vegeta says that to become a Super Saiyan, you must have a pure heart (pure evil or pure good doesn't seem to matter) and be in a state of pure rage, with (except for SSJ3, which is achieved off-screen) the new levels involving them getting more enraged than they have ever been before. While this doesn't last, and any future transformations allow them to stay 'normal' it is still pretty unclear how changed they are as a result of this brief spurt of maddening rage. Basically, at each newer Super Saiyan level, they tend to revert to the typical Saiyan brutality. Vegeta doesn't have any similar problem because extreme brutality is his default state.
Gohan is truly shocking in his beatdown of Cell as he goes from restraining himself because he doesn't want to fight to torturing Cell in the space of a few minutes. Even Goku is shocked by what the transformation does to his son.
And in Dragon Ball GT, Goku loses his reason and turns into a giant, supremely powerful golden ape when he first transforms into a SS4. He Got Better.
That was simply the latest in a long list of giant ape transformations from Dragon Ball and early DBZ, which had the same effect: multiplying the Saiyan's power level by a factor of 10, but causing their minds to become animalistic. Vegeta was the only one to remain sane in this form... presumably due to actually being trained in its use.
Frieza's reason for using transformations to hide the bulk of his power is heavily implied to be due to losing what little sanity he already had if he went all out.
In Bleach, Ichigo's powers are greatly amplified when his inner Hollow is released, but he suppresses it in order to preserve his sanity. Even then, suppressing it didn't work in the long run. It takes a major Battle in the Center of the Mind for him to gain control over it.
It's worse than that. During the third battle, the Hollow wasn't trying to hurt Ichigo, his goal was to make the hero accept him as part of his soul instead of suppressing him. Once Ichigo did it, he became a Physical God. It doesn't seem to mess with his head much, though.
Aizen has never been entirely sane, but his insanity slowly becomes more and more obvious the greater his power becomes. By the time he's fused with the Hougyoku, he's fullblown crazy.
Also, Kouga (from the third anime filler arc) fits this trope perfectly. His power (forcing others' zanpakutou to do his bidding) is truly enormous; and he quickly goes insane. Kouga had plenty of external help on that trip to insanity, but his obsession with his own great power left him a little unbalanced even when he was still one of the good guys.
Itsuki's Glam Sight in Rental Magica gives him his Crouching Moron, Hidden Badass ability that makes him just the right sort of leader for his team of mages. However, the more he uses it, the more it eats away at his sanity. Thus, Honami and the others warn him not to use it as much as possible.
InuYasha's heritage of demonic power from his Greater Demon father is so strong that it is too much for his half-human body to handle. That's why his father created the Tessaiga, to serve as a Restraining Bolt on Inuyasha's demonic power and thus preserve his sanity (while also making up for the power suppressed with the sword's own usefulness). If the sword is taken from Inuyasha or broken, he must refrain from getting too emotionally excited, or he risks unleashing his full, uncontrollable strength. Worse, each subsequent overload renders him more insane than the last, and it becomes harder to snap him out of it. If left unchecked, Inuyasha would eventually be reduced to a mindless monster killing and destroying everything and everyone around him, permanently. Initially, the sword was effective in this role no matter how far away it was, but after it was broken the first time apparently the sealing effect was weakened and even after it was repaired it has to be kept close at hand at all times.
Contractors in Darker Than Black appear to have 'complete lack of conscience' as one side-effect of gaining their powers (and their powers are usually destructive in nature). Even the sanest amongst them are Affably Evil or Anti Heroic at best and have no problems with taking lives, though whether it is the powers themselves that cause it or the result of the extensive masquerade surrounding them and how their fellow humans treat them is up to debate.
Human-type homunculi of Busou Renkin are a borderline example, as while becoming a homunculus does grant a human great power, none of the ones seen seem like they were particularly sane beforehand. Victor, though, is a dead-on example, as becoming a Victor made him do a Face-Heel Turn from an alchemic warrior to a demigod attempting to destroy all alchemy.
No, the cause of his Face-Heel Turn was his superiors turning his thirteen-year-old daughter into a homunculus and sending her to kill him.
In Chrono Crusade, Joshua Christopher is given the horns of a demon by the Big Bad. The power is too much for the small boy, however — within minutes he's gone completely insane, using his powers to freeze everyone around him in time and destroy the orphanage he lived in. It gets so bad that later he can't even remember his own sister.
It's even worse than that in the manga. It turns out that a demon's horns, in addition to granting them direct access to the Astral line, are also what connect them to their "mother", Pandemonium. That constant, painful noise that Joshua was complaining about? It was an Eldritch Abominationconstantly mind-raping him.
In the anime, though the horns did damage his mind on their own, the "noise" he complained of was the thoughts of every human around him. In other words, uncontrollable telepathy that forced him to hear what every single person around him was thinking, all at once, all the time. No wonder he went mad if he had to be near people.
Due to metatron poisoning, Radium of the Zone of the EndersOVA, Idolo, goes completely off the deep end after spending just a moment too long in the cockpit of the Idolo. This does not end well for anyone. The same can be said for Ridley Nohman with his Anubis, from a rather immoral rebel leader (in Dolores, I) into destruction-obsessed guy (ZOE: The 2nd Runner).
In Devilman Lady, the entire story centres around evolution and the gaining of vast, beast-like powers by ordinary people, many of whom go insane. The main character, Jun, for much of the series appears to be descending into insanity herself despite her best efforts not to. The climax moment of this aspect of the series is when, having been prevented from indulge her bloodlust by the Human Alliance, she attacks a nurse, but ultimately overcomes her instincts by drawing her own blood.
In Tenjho Tenge, characters who possess supernatural powers are remarked as always being in danger of becoming insane. A classic example of this is Natsume Shin, Maya and Aya's elder brother who was overwhelmed by his powers and started killing random people. This leads to powers being referred to as "Dragons" that will devour their wielder's sanity. Because of this, there is a tremendous social stigma attached to the possession of supernatural abilities, which naturally only serves to aggravate the problem even more.
s-CRY-ed shows this one off pretty well, with most (if not debatably all) of the Alter Power users being completely insane to some degree. Particular samples include Straight Cougar, most of the one-shot villains, and our main Kazuma (whose personality initially flips between Jerk with a Heart of Gold and nice-guy… only for the nice guy to completely vanish by the final battle). It's worth noting that both Cougar and Kazuma are rare heroic examples of this trope. They're clearly bonkers, but they're still the good guys.
A few of the characters in the Mai-HiME mangaverse are given special earrings created using SEARRS technology, which allows them to summon more powerful CHILDs than any of the HiME, and they don't need an emotional anchor to use said powers. However, relying on this ability too much can drive them insane.
A Claymore who activates her demonic powers (i.e. "Awakens") gains great power but stands a chance of losing her humanity and turning into a human flesh-craving demon permanently.
In Rosario + Vampire, being injected with a youkai's blood gives you all of their abilities for a short while. Eventually, the effect starts wearing your body down. One time too many will kill you (if you're lucky) or horribly mangle your body and, if vampire blood is involved, leave you a mindless killing machine. Fortunately, there are ways to counteract the less-than-desirable effects. Subverted with Tsukune, in the fact that he doesn't even have a mind anymore when this happens.
In Get Backers, Ginji and Kazuki have this as a side effect of their Superpowered Evil Sides. Ginji's "Lightning Lord" aspect is quiet, cold, and utterly ruthless, while Kazuki goes absolutely berserk when he releases the seal on his "Stigma," becoming vicious and blood-thirsty. They're always sorry afterward.
In Soul Eater, the characters infected with Black Blood have their strength increased tremendously when they use it, but also run the risk of losing themselves completely to insanity. Oops.
Also the result of misusing the Nakatsukasa Purpose, which also drives you mad whilst killing you slowly (it damages your soul).
This is also the reason why witches are so dangerous in the series. Up till a certain age, they're normal. But then the "Sway of Magic" affects them and causes them to be evil and destructive. However, there are a few exceptions where this doesn't happen due to the witch having their mind on something else other than their magic. In addition, those who possess magic which has little in the way of destructive capabilities are much more resistant, if not outright immune to the Sway: Kim Dheal, for example, has healing powers and is more or less immune.
Black☆Star and Death The Kid actually use it to their advantage.
Several characters use insanity to their advantage, but most do so from outside sources where they are at risk of permanently losing their minds. Even the Nakatsukasas trace their power back to Arachne's experiments. Kid's is a unique case so far because he is an Anthropomorphic Personification of a kind of insanity, albeit a deliberately weakened and incomplete one (a 'fragment of Shinigami').
Played with in Mirai Nikki, where some individuals who're not so mentally sound to begin with are given the ability to predict the future and told by God to kill each other Highlander style. Whether the ability, situation, or just generally being that nuts before drives them crazy is up for interpretation.
Pokémon Special uses this in the R/S arc. The Red and Blue Orbs can be wielded by mortals to control Groudon and Kyogre, but sufficient willpower is required to prevent loss of self. One Magma Elite tried and was driven mad; Archie and Maxie were so consumed by their lust for power that the Orbs wielded them. To prevent the same from happening to their (unknowing at the time) new hosts, Juan, Liza, and Tate used the temporal abnormalities of Mirage Island to temper Ruby and Sapphire's collective resolve. Ruby foolishly coaxed the Blue Orb from Sapphire for his own use, but he turned out to have enough strength of will to calm both titans once more.
The anime has Oakley in Pokémon Heroes, who loses it and tries to destroy the world once she gains control of a machine powered by Latios. Even her partner Annie is startled by the change.
The anime also has it in the form of Korrina's Lucario. It's unable to control the power of Mega Evolution, and thus whenever it Mega Evolves it goes insane, though it's training in an attempt to gain control over itself.
In Naruto, Sasuke seems to grow progressively more irrational the stronger he gets (as, to a lesser extent, do Itachi and Obito). Eventually it's revealed that this is part of how their powers work; whenever a member of the Uchiha clan experiences strong emotion, it creates a burst of energy that increases the power of their Sharingan… and also damages their brain. In short, the best way for an Uchiha to become extremely powerful is to Mind Rape themselves into oblivion, a lot.
The Sage of Six Paths explains that this happened to his mother and Madara when they gained the power of the God-Tree. Not coincidentally, both struck upon the idea of executing an Assimilation Plot where they would effectively become gods because a servant of Kaguya had deliberately planted information to recreate said plan.
Fairy Tail: This was the result of Elfman's failed Take Over spell against a monster called The Beast a few years before the story started. In order to save his older sister, he used his magic on a gigantic magical monster, but wasn't quite strong enough to control it. He saved his older sister, and killed the younger. Out of fear of this happening again, he stopped using full body Take Over magic (eventually, he was able to control his power).
By the time Natsu fights Laxus during the Battle of Fairy Tail arc, Laxus has officially become so mad with power, he loses all reason, and tries to destroy everything and everyone.
Both played straight and averted in Digimon. On the one hand, you have Digimon like Omnimon, who are immensely powerful, but perfectly sane, noble, and benevolent. On the other hand, you have Beelzemon and Diaboromon, who are driven only by their desire to gain power and cause destruction.
In the original Fullmetal Alchemist anime, Ed gets a power overload from contact with a huge amount of unfinished Philosopher's Stone and goes full-on uncontrollable with so much power it is described that if this goes on, he can attain enough power to be god-like (even freaking out the Homunculi) until a Cooldown Hug snaps him out of it.
In Puella Magi Madoka Magica, almost all magical girls lose it in varying degrees due to a combination of having awesome power and still not getting what they wanted, having to fight constantly and lose any semblance of a social life, having their souls sucked out and put in a gem, and then finding out that the enemies they've been fighting all this time used to be magical girls themselves. Prime examples include Sayaka, and Mami in another timeline. More to the point, witches are magical girls that have succumbed to despair and insanity.
If piloting a Super Robot counts as "Great Power", then Neon Genesis Evangelion speaks for itself. The pilots, Shinji particularly, go mad from the experience of piloting the Evas.
The witches of Witch Hunter are a bag of mixed nuts. The magic powers they wield have different affects on all of them. Some stay the same sweet person they were before, the only change they undergo being the magic they now wield and their now immortal bodies... Others not so much. The main character's sister went completely insane upon gaining her powers, murdered her own father, her brother's beloved mentor, and countless tens of thousands of people and only approaches her normal mentality as she uses her magic too much and starts running out. Almost all witches introduced who are not on the side of the humans are sadistic and cruel, viewing human life as something deserving of only destruction, despite all of them having once been human.
In Fate/Zero and Fate/stay night, both Berserker got Mad Enhancement, a Class Skill that trades one’s sanity for added power.
One of the rare good endings for someone getting the Super-Soldier serum is Isaiah Bradley, grandfather of Patriot from the Young Avengers. Instead of going crazy with power, he simply lost a lot of his intelligence, sort of a super-Alzheimer's. Yes, that's what passes for a good ending when trying to reproduce the Super-Soldier serum. One wonders why they keep trying.
In almost every incarnation of Spider-Man, when he gains access to the power-enhancing abilities of the symbiote, he ends up becoming irrationally angry and cocky. Or, in the case of Spider-Man 3, an emo.
Spidey's Arch-Enemy, the Green Goblin, in his original portrayal, was this. He was a distant father whose business practices were not always scrupulous, but he had redeeming traits, such as genuinely loving his son, and saving Gwen Stacy's father's life. Then the formula that gave him his powers drove him insane, though periodic bouts of amnesia restored him to his former self. After he killed Gwen Stacy, however, he was rewritten as always being sociopathic, with the kinder personality that he possessed during his periods of amnesia being a false personality. The Goblin formula probably enhanced his insanity, however.
In Hellboy, losing control of one's powers and/or humanity was a major theme of the series, especially for Liz Sherman and Hellboy himself. That is to say: Liz burned her family to death by accident as a kid, and Hellboy doesn't and can't and won't use his flashier superpowers, since they mostly involve ending the world, but occasionally they get stolen, one way or another. His crown, his Name, his right hand, ALL THE BLOOD IN HIS BODY...
In Gold Digger, dragon hybrids between the tribes (Platinum, Gold, Copper, and Iron) are extremely rare and extremely powerful. Every single one has gone insane, however, with the sole exception being one of the comic's supporting cast, D'bra. And then, most dragons believe her temper is a sign that it's only a matter of time.
Apparently, Chris Claremont likes this one, or used to. In X-Men, The Phoenix being a cosmic entity was a Retcon to satisfy the then-editor-in-chief's requirement for bringing Jean back: she had to be innocent of her crimes as Phoenix. (The destroyer of five billion lives couldn't very well be welcomed back to the team with open arms. In fact, her original Heroic Sacrifice was mandated for that very reason.) The original story portrayed Jean's cosmic powers as the ultimate expression of her abilities, and the change from hero to Anti-Hero to cosmic-scale threat as simply the result of having the sort of powers she now possessed. Storm also began a similar change upon maxing out her powers, but thankfully was able to return to her previous self (her power level returning to normal with it) within that issue and before she did anything particularly heinous.
The 'return' of Jean Grey in the X-Factor retread of the original X-Men was so badly done that it left permanent damage to the storylines of the Marvel Universe. The obvious moral cop-out of 'it wasn't really her' not only undid the basic point of one of the landmark storylines of the MU, but was done in a half-assed way, because they tried to claim that Jean deserved the credit for the self-sacrifice of the Phoenix entity, but not the blame for its crimes, even though both supposedly derive from the human element from Jean. Sorry, folks, you can't have that both ways.
To make it even worse, Claremont clearly hated the idea that the Phoenix entity was not Jean, he kept trying to sneakily re-retcon it back, and the story got into the hands of other writers and mutated further, it eventually became a total, unworkable, self-contradictory Continuity Snarl.
Oddly enough, though, Jean's time traveling daughterRachel, who already had ample reason to have gone insane (but didn't) before acquiring the Phoenix power, managed to wield it for years without going crazy. And then lost the power (despite the Phoenix itself insisting that it had permanently merged with her).
Long story short, Jean's power level, mental state, and the effect these have on each other all depends on where Jean ends and the Phoenix begins. Too bad no two mentions of the Phoenix in a row give the same answer on that score. However, the cosmic critter isn't malevolent. Mastermind screwing with her head was what turned Phoenix into Dark Phoenix, and to suggest it might happen again much later in Phoenix: Endsong, it took a botched resurrection to again put the Phoenix out of whack. Poor Jean will Never Live It Down, despite this.
Claremont also established that classic X-Men adversary Magneto's magnetic powers damage his sanity over time. This explains rather a lot; wouldn't being able to control one of the four fundamental forces of the universe screw you up, too?
Some writers have tried to pull the same thing with Havoc (Cyclops' brother), who controls (or at least, has a degree of access to) the Power Cosmic, another fundamental force of the Marvel universe.
Magneto comes close to saying this trope by name in issue two of the nineties X-Men series. When Moira MacTaggert explains how his powers played havoc with his mind, he states: "What, with great power comes mental instability?"
Magneto's daughters, Polaris and the Scarlet Witch, suffer from similar sanity-damaging "cursed" powers. Insanity might be In the Blood where this family's concerned.
It's implied that the Scarlet Witch's crazy came from the Mind Screw the Avengers did so she'd forget her maybe/sort-of/magic children rather than from her powers themselves. Similarly, Polaris is only crazy when outside influence is involved... it just happens to her more often than it does to anyone else.
This was lampshaded in the Assault on Weapon Plus story arc, where the Weapon Plus files stated that super soldier experiments on criminals and psychopaths yielded less than reliable results, prompting them to find a different method of creating anti-mutant super soldiers.
Knuckles' ancestor Dimitri ends up becoming the insane Enerjak after absorbing 11 Chaos Emeralds worth of power, one of the most powerful and evil villains in the series.
Knux's dad Locke, obsessed with the prophecies surrounding his son, genetically engineered himself and infused Knux' egg with energy from the Master Emerald in an attempt to fulfill those prophecies. They're more or less estranged, now, because of this.
Knux himself isn't safe from this, having appropriated Dimitri's mantle of Enerjak to avenge his race's decimation at Eggman's hand, only to quickly degenerate into "technology = evil" and attempt to wipe out all cybernetics from the planet, including those of the Dark Legion and his own girlfriend Julie-Su. This was caused by Dr. Finitivus, a scientist who tried to drain Knuckles' Chaos power earlier, only to have it backfire and transform him. He then decided to go and screw over the entire echidna race by deciding everyone needs to die in a fire.
In the (non-canon) "25 Years Later" storyline, Knux ends up becoming "Chaos Knuckles" (a form he took up in the normal canon without much ill effect), and ends up trying to change the world, as well. The result was the almost complete destruction of his friendship with Sonic and the loss of his right eye.
In the future of an alternate universe, Knuckles not only tried to change the world, but he succeeded in conquering it after harnessing the power of all the emeralds. He ripped the souls out of nearly all the main characters (Eggman, Sonic, Shadow, Tails, his own mate, etc.), sunk a continent, and then decided to try and amuse himself by conquering other dimensions. He uses the souls of his victims to create his Prelate Army. Oh, and he took on the name Enerjak in this timeline because "Enerjak the Hero" sounded cool.
Some versions of Sonic's Super form are like this, becoming a Superpowered Evil Side. This is mainly seen in the British Fleetway comics, where Sonic always becomes a psychopath when he changes to Super Sonic.
Such as the third season of Sonic X. Seeing a hostage Chris and Cosmo traumatised in a glass cage gives a view of how upset Sonic gets when his friends are harmed. Combined with the 500-odd fake Chaos Emeralds nearby, Sonic turns Dark Super, shredding two test robots (based on speed and power respectively) in the blink of an eye, complete with the slightly unhinged giggle and a "Alright, Let's try 'em out!". It took a minute and Eggman's logical prose to snap Sonic out of it, surprising somewhat as they've been at each other for years, and Sonic could quite easily rip Eggman to gibs.
In Archie canon, Super Scourge (Scourge being an evil version of Sonic who is already hyped on Master Emerald energy, so this makes him even worse than a standard super form), upon seeing that his own world (which he conquered) nor Sonic's own will accept him as King, says he's going to spindash both worlds in half just because he can, and will do this to one world after another until one gives him "the respect he deserves."
Sonic from the games actually becomes kind of crazy when he turns into Super/Hyper Sonic, at least in the old-school games on the Genesis. Suddenly goes from fast and cocky but vulnerable to fast and cocky and invulnerable, tearing through anything in his path. And way, way harder to control.
On Mobius Prime, when Amy Rose used the Ring of Acorns on herself, she became her Sonic Adventure self. On the Mirror Universe world of Moebius, when Anti-Amy did it, she went certifiably insane.
Matthew Cable from Swamp Thing. His Psychic Powers and his spiraling alcoholic insanity both stem from the same illicit electroshock treatments.
Warren Ellis' Global Frequency not only invokes this, but explains it in terms of the surgical alterations required to keep the superstrong bionic arm or whatever from physically tearing the body apart. "They gave him a mirror."
In newuniversal, which is also by Warren Ellis, John Tensen gains telepathic powers that let him "see" a person's misdeeds or ignoble intentions. The first time he used these abilities, he discovered that his own nurse was planning to poison him. Tensen, not surprisingly, became Ax-Crazy and is now the "worst serial killer in New York City history," to quote a minor character.
In Powers, a naturally occurring variant occurs in "The Sellouts" storyline, in which a never-aging Captain Ersatz of Superman gradually loses his connection with humanity and goes insane, declaring himself to be a God. It's discussed that this may partly be a result of his ever-increasing level of superpowers, which go way beyond anything seen before (to the extent that the government has lied about exactly how powerful he is in order to prevent hysteria about him), and partly because, despite the fact that he doesn't look very old, he's at least over a hundred years old and has gone senile.
Deadpool was probably messed up before developing terminal cancer, but the Weapon X program (which initially failed to give him a Healing Factor) gives him a hard shove in that direction. Then Dr. Killebrew experiments on and tortures him to the point of having visions of (and falling in love with) Death. What finally demolishes his sanity is when Killebrew orders him killed, his healing factor finally kicks in, saving his life, making his disfigurement permanent, and causing Death to reject him. Depending on the writer, he's a mix of Ax-Crazy, Deadpan Snarker, gleeful Genre Savvy, and Medium Awareness.
The newest Kryptonite Man is a scientist who thinks Kryptonite (there is now a lot of it on Earth) can be used as a safe energy source. When he himself becomes that energy source, he decides to show the world how effective it can be by... a murderous rampage. That never ends well. Later, in the same storyline, another scientist goes cuckoo bananas when he gains control over an oversized amoeba. Or so it seems. Superman subdues the guy, who says he didn't want to do it, but Intergang, a powerful criminal organization, made him.
Retconned for Doctor Magnus, the leader of the Metal Men. He needs a careful application of medicine in order to stay stable and good. He's kidnapped along with genuine mad scientists and they cancel his meds, intending for him to regress to his previous level of insane creativity, in which he created a horrific weapon of mass destruction, the Plutonium Man. Though he does recreate the Plutonium Man, he destabilizes very quickly, and with the help of several sentient robots he'd managed to cook up in his lab, invents a gun with living ammo and goes on a rampage (against evil men only), screaming about how he really needed his meds.
Handled interestingly with Marvel'sSentry. His powers are like some ridiculous combination of Superman and Franklin Richards, making him technically unstoppable and all-powerful. He's also a paranoid schizophrenic who managed to convince himself that there was an evil galactic power called The Void that would destroy the earth if he stayed a superhero... and then actually created it out of thin air, making a problem for The Avengers to handle while Emma Frost gave him some emergency psychotherapy. During this time, he also managed to Retcon himself out of his own universe, so that his Golden Age exploits all became some comic writer's fantasy. In something of a subversion, Sentry's not a villain: in current canon, he works to use his powers for a great deal of good. Unfortunately, his psychosis still isn't fully under control, and it's a disability that is sometimes just impossible to work around.
Later developments suggest that the Sentry is an inversion of this trope. Robert Reynolds was already a mentally unstable drug addict before taking the serum that gave him his powers, so it's more like someone with great insanity given great power. Furthermore, Reynolds didn't become the Sentry and create the Void, it was the other way around...
An alternate interpretation is that Reynolds splits into the Sentry and the Void, neither of which are more physically or psychologically 'real' than the other.
Incredible Hulk. The gamma bomb gave Bruce Banner huge power and exacerbated his multiple personality syndrome. Plus, the madder he gets, the stronger he becomes.
Interestingly, for both The Mask and Marvel's gamma ray mutants, what happens to the subject's mind depends on what part of their personality they had dissociated themselves from. Most people who get the Mask unlock their evil side, but the fellow in The Movie didn't have an evil side, only a chaotic side, so he essentially became a Looney Tunes character. Marvel goes into more detail — Banner suppressed the rage that came from being abused as a child, She-Hulk suppressed her sexuality, Doc Samson suppressed his desire to be a hero, and the Abomination suppressed his self-hatred.
Interesting case with Black Adam from DC. His powers don't drive him crazy, but he can share them, and anyone who would take them on immediately turns evil. Anyone, including the goddess of love.
When the Canadian government was looking for people to join Alpha Flight, they initially had trouble finding recruits. The people in charge of the program decided to try creating their own superbeings, and they got the bright idea to experiment on a Serial Killer who got a pardon in exchange for agreeing to participate. The result was a crazed monster with deadly psychic abilities calling itself Bedlam. Wolverine had initially signed on to join Alpha Flight, but this debacle was what caused him to leave the group in disgust.
In Zenith, the alternate Earth Maximan, who had been superpowered since the 40s, ended up going completely insane and killing everyone on his version of Earth. That said, most of the '60s superheroes became somewhat less than rational by the time of the early '90s.
Arguably completely inverted in All-Star Superman: Lex Luthor temporarily gains Superman's powers, and while he predictably rampages, he finds himself stopping to examine the amazing perspective his newfound powers and super-senses give him. He eventually concludes that having the level of power and insight that Superman does would make people care for their fellow human beings, and mellows out considerably after losing the powers.
Similarly inverted (or not) in The Authority, where the Doctor gives up his powers to a previous Doctor, who was relieved of his duties for being a psychotic maniac. As soon as the full extent of his powers kicked in, he was overwhelmed by the empathy for every living thing in existence. Super-empathy being part of the Doctor's role as the world's shaman, makes you wonder why this guy was psychotic in the first place.
Every Doctor gets a new power, added to the ones the previous Doctor who had the job had all the way back to the first one. After he went psycho, the next guy got super empathy, which he used A LOT of heroin to deal with.
A rather mundane case of this appears in Incorruptible. Max Damage becomes stronger and more invulnerable the longer he stays awake. If he stays awake for say, a week, his powers reach Physical God levels. However, he is still vulnerable to the effects of sleep deprivation. The result is someone who is mentally exhausted and strong enough to topple buildings — an unnerving combination. Dealing with the numbness his invulnerability brings with it for years has also taken its toll on Max's sanity. Given that the premise of the story is Max's attempt at a Heel-Face Turn to save the world from an evil SupermanExpy, this causes even more problems. He needs to be strong enough to fight the Plutonian yet not let his insanity push him back toward a life of evil.
An interesting case concerning Azrael. When Jean-Paul Valley was born, he was implanted with a brainwashing trance of sorts known as The System, which would grant him amazing strength and agility when he donned the gear of Azrael, though it made him Brainwashed and Crazy, pushing him to want to kill the guilty. When he first abandoned that role, he functioned pretty well... until the Scarecrow doused him with Fear Gas when JP took up the Mantle of the Bat in Bruce Wayne's place. The entirety of the subsequent Knightquest storyline has Jean-Paul constantly fighting The System until he lets Abattoir die. When he does, he finally submits to The System and becomes a fearful mixture of Batman and Azrael and forces Bruce to take back the Mantle.
Hunter Zolomon spent his life in a Trauma Conga Line, but he remained a good cop until Gorilla Grodd crippled him and the Flash, whom Hunter considered a friend, refused to use time travel to undo the damage. Hunter attempted to use the Flash's cosmic treadmill himself, but it blew up in his face. The resulting super powers had the side effect of scrambling his thought processes, until he decided that the best way to help his friend the Flash was to make him a better hero through tragedy as Zoom. Hunter still thinks he's helping the Flash, although he gets occasional flashes of My God, What Have I Done? when Wally foils his plans.
W.I.T.C.H. has the Heart of Kandrakar, an artifact of immense power that may drive its Keeper mad just by being that powerful. While the protagonist, Will, is a sane and well-adjusted person and her extreme jealousy has a good Freudian Excuse, the Big Bad of the second story arc was the previous Keeper and has been driven mad, and Will has the occasional moment of slight (and useful) madness.
Fallout: Equestria: The Goddess, as if the self-given name wasn't a dead giveaway. She is an immensely powerful gestalt psyche that commands the alicorns of the Unity. Unfortunately, being a disharmonious merging of some of the early-war era's most powerful unicorns (including Twilight Sparkle and Trixie, the latter being the first victim and the most dominant persona) means that she's not at all mentally healthy.
In Project Horizons, we find that the Goddess likes to take every bad memory, every indication of a shortcoming or flaw, and shunt them into the head of Lacunae, an exiled alicorn whose position at Hoofington means she's not fully connected to the Unity and can be tuned out. Which means that the Goddess won't grow and improve in any way, although Lacunae is becoming a better pony from it.
Red Lightning: Insanity is almost a requirement to have a superpower.
The Infinite Loops is about a bunch of fictional characters reliving their stories over and over again. They gain incredible powers and also incredible neuroseses. In fact, this trope is referenced by name.
Princess Twilight Sparkle, the Fourth Alicorn, in Pink Alert 3. Ten years of authority have transformed her into a paranoid and aggressive nervous wreck who is little more than a Mad God to Equestria's citizens. She owns a private evil corporation, constantly accuses those around her of being Communists, is so magically unbalanced that she can psychically explode ponies' heads by sneezing too hard, and has a heated argument with an unconscious clone of herself floating in a biogel vat. And all that's just her introductory bonus chapter.
Bananas: After overthrowing the government of San Marcos, rebel leader Esposito declares himself the new president. Esposito also announces "the official language of San Marcos will be Swedish", "citizens will be required to change their underwear every half-hour", and "all children under 16 years old are now 16 years old." He spends the remainder of the film in an insane asylum.
Jean Grey in her manifestation as Dark Phoenix in X-Men: The Last Stand. Although it is explained that she was already mentally unstable as a girl and her massive powers had to be reduced for her own good and that of everyone around her by putting mental blocks into her psyche. When these were removed, she started killing people with her mind. note (This is actually closer to the original Phoenix story than the later comic and adaptation stories that portray Jean as having been Touched by Vorlons; see the Comic Books section above.)
In RoboCop 2, the evil corporation OCP attempts to build a successor to Robocop, but in all cases the new cyborg goes crazy and commits suicide. Finally, they stick the brain of a convicted, drug-addicted, psychopath into the cyborg. That doesn't work out too well in the end, either.
Honestly, you have to wonder what made them think that turning a convict into a heavily-armed weapons platform was a good idea.
Ironically, the scientists theorize that Robocop was a success due to the very qualities that made Murphy a good cop in life: his highly Catholic upbringing and stable family life imbued him with a strong moral compass, selfless devotion to duty, and an aversion to suicide. Then they turn around and choose a murderous psychopath, because they thought that they could control him through his drug addiction. Umm... Nope.
That was more of an Enemy Without, though. Morbius seemed sane right up to the end.
Morbius may have had a mental block preventing him from understanding what had happened, because it was simply too horrible for him to accept that the monster was an inherent part of himself, which meant he had killed all of his friends.
In Spider-Man, this is the origin of the Green Goblin. The process to make him a super soldier also seems to produce a homicidal second personality. Ironically, Norman subjected himself to the serum in an effort to prove it would not have that effect on humans...
Doctor Octopus in the second film was made crazy by the robot arms. In fact, they slaughtered a room full of medical personnel while Otto was still unconscious. He overcame their programming just in time for a Heroic Sacrifice.
Same with Flint Marko in the third. He wasn't an especially good person before the accident that turned him into Sandman, but afterward he was just nuts. Eddie Brock, on the other hand, started out as a psycho; gaining the Venom symbiote just allowed him to express it in new ways. Flint Marko was desperate, which led to all the bad things he was involved in (including his part in Ben's murder). He needed money desperately to save his daughter's life, and would do anything to get it.
The Incredible Hulk: The most recent movie finds hardened veteran soldier, Emil Blonsky, given a prototype super-soldier serum — to put "what I know now into the body of a man ten years younger". The combination of being defeated despite this and the taste of such power grow into the classic Comes Great Insanity.
Funnily enough, that serum is all but directly stated to be the one that gave Captain America his powers, but with Blonsky lacking the additional radiotherapy, his insanity is a classic symptom of the Super Soldier serum gone wrong. Then he demands (against repeated warnings) a dose of an even more experimental serum from a much less reputable source and... let's just say the end results of that little cocktail ain't pretty.
Incidentally, in the comics this was power born of suppressed madness, which that power incidentally un-suppressed — he was a gamma ray mutant like the Hulk, and they all get personality shifts depending on what part of their personality they're suppressing. He had some serious issues, and they manifested brutally.
Universal Soldier. In the sequel, one of them even gets an artificially intelligent, Self-Evolving Thought Helix military supercomputer downloaded into them.
Hollow Man, starring Kevin Bacon. He goes nuts after gaining his ability because he realizes he can get away with a lot of crimes while invisible. It goes from disgustingly creepy (opening a sleeping co-worker's top) to badness and murder real soon. There's some talk about the invisibility Psycho Serum causing insanity, but it's never made clear how much of an effect it's supposed to be having on him.
It's amazing what you can do when you don't have to look at yourself in the mirror.
As expected, the failure with Bacon's character doesn't stop the government. In the sequel, they use the serum on several more people, including a decorated soldier (Christian Slater), who also goes insane and starts killing people. Unfortunately, his soldier training makes him doubly difficult to kill. Unlike the scientist, who already had quite an ego, the soldier goes insane from a side effect of turning cells transparent. Since the skin no longer protects the brain from solar radiation, this causes unavoidable mutations and, as a result, insanity.
In Serenity, River Tam is a paranoid schizophrenic who suffers from hallucinations, delusions, post-traumatic stress, identity and memory disorders, and unfiltered emotional responses. All of this is due to government experimentation on her brain that gave her uncontrolled empathicPsychic Powers that tie in with implanted combat abilities that make her the single most devastating weapon in the setting.
Peter Drak: Power doesn't make you good, David. It just makes you powerful.
Subverted in Amazon Women on the Moon, when Ed Begley, Jr. plays the son of the original invisible man. He creates a potion that he believes will turn him invisible but not insane. Unfortunately, he becomes insane but visible.
The Mask: Happens to Stanley Ipkiss whenever he wears the Mask (and when Milo puts it on as well). Dorian Tyrell, not so much. As explained above, the Mask unlocks the suppressed part of a person's psyche. Tyrell, a unrepentant criminal, already had all his nastiness full on the surface, so the Mask just made him invincible and monstrous.
Titan in Megamind mixes this with great immaturity.
Similar to Kefka Palazzo, it is hinted in Captain America: The First Avenger that Johann Schmidt was the first person to receive the supersoldier serum, but for various reasons, namely a combination of the serum not being perfected yet as well as his dark inner nature, he received a tremendous power boost at the cost of his sanity (and his human form). Steve Rogers himself, however, is selected for being an Ideal Hero, and thus will be responsible with his power.
Averted in Lucy. The title character stays rational, cool, and focused (perhaps getting even more so) as her power grows to godlike levels.
One theory why Epics are all sociopaths in The Reckoners Trilogy. Turns out it's true. With the exception of Transference Epics (aka Gifters) who give their powers away, any Epic who uses their powers will turn into a hateful, angry sociopath in minutes. Even people in close proximity are not immune to this — it's implied that Epic powers, even when Gifted, have a high probability of causing the same arrogant amorality in other people when used by them instead. One character alludes to a time when the police of his hometown joined with an Epic and "The good ones [left the force.] The bad ones stayed on, and they got worse." The main character, David, finds what's secretly gifted Epic power to be addictive, and nearly believes that the Reckoners should take over the infrastructure of Newcago for what would surely be a Full-Circle Revolution.
In the Hero.com series, along with its sister series ''Villain Dot Net'', when a Prime, someone born with powers that doesn't need to download them from the titular websites, downloads powers from them, it either causes insanity or death.
There are also the six Core Powers, the original powers from which every other power in existence is only a weakened, mutated descendant of one or more of these powers, which can only be wielded by one person at a time (though 2 of them were divided into segments that different people could use at once, albeit in a weakened form). Of the six, 3 are known, a Time Master power, wielded by the villainous Lord Eon, power over life and death, and a Gravity Master power. The Core Powers can corrupt anyone without the strength of will to resist the lure of their sheer power, with only a few characters being immune.
Saidin, the magic used by males in The Wheel of Time, is tainted by The Dark One, causing inevitable insanity in its users. As time progresses, one of the main characters begins to show the effects of this, becoming schizophrenic, moody, and temperamental; halfway through the series, he seems like a completely different person, though he isunder a lot of pressure... The Forsaken also have access to what they call the True Power, an extremely addictive, evil flavor of magic that also has serious psychological consequences; most would only consider using it under dire need.
Possibly exemplified best when some poor soul using Saidin breaks down AFTER ONE DAY, and starts screaming that there are spiders under his skin.
It's also worth mentioning that Saidin use can bring about other lovely effects, such as rotting flesh. It is entirely random as to which will affect you first, when, and to what degree.
Played straight and later justified in the third book with the Lord Ruler, a main villain.
Also true for the Lord Ruler's Dragons, the Steel Inquisitors. The Inquisitors use hemalurgy, a ghoulish form of magic that allows them to remove portions of someone else's lifeforce by killing them with a metal spike, trapping said life force in the spike, and then impaling *themselves* with said spike to acquire whatever power they stole. This makes them supremely powerful, but is in no way good for their long-term mental health. All the Inquisitors we see in the series are somewhat... homicidal.
It doesn't help AT ALL that hemalurgy provides an "in" for the series' Big Bad to mess around with people's minds and in the Inquisitors' case outright control them.
The One Ring in The Lord of the Rings slowly destroys the sanity of whatever schmuck has got a hold of it, first by making them obsessed with it and unable to let it come to harm, then tormenting and tempting them with visions of power they probably can't achieve, and finally devouring their mind. The other Nine Rings wrecked their bearers as well, giving them Age Without Youth until every moment of existence is an unbearable torture, and making them unable to disobey Sauron's commands even for an instant. The Seven probably aren't very safe either, though Dwarves seem to be mostly immune to the effects. Really, only the Three (which Sauron never touched) aren't liable to drive a wielder crazy, and that's only unless/until he regains the Ruling Ring.
Jacen Solo in the Star Wars "Legacy of the Force" novels seems to get crazier and crazier the more he falls to the Dark Side of the Force. He first justifies his actions as necessary sacrifices for the good of the galaxy, but by the end of "Fury," he uses the Force to break an underling's neck for failing him. They don't call it The Dark Side for no reason. The same applies to Anakin/Vader and a host of other Expanded Universe characters.
In David Brin's original novel The Postman, the brutal survivalists/Holnists are led by General Macklin and his aides, who were pre-war experiments on creating soldiers with superhuman strength and speed. The government chose the most ruthless, intelligent, and efficient killers in its military, with foreseeable results when the US itself turned into an anarchistic warzone. Macklin is finally killed by George Powhatan, a later experiment of the same ilk, though with a nature loving Neo-Hippie as subject.
Midnight by Dean Koontz. A rather twistedscientific genius has designed microchip-like spheres that augment a person's mental and physical abilities, but suppress all their emotions except fear, which produces some rather odd behavior on its own. Then the townspeople begin to discover that an accidental side-effect gives them mind-over-matter shapeshifting powers, and they promptly escape into forms in which their lack of emotions doesn't bother them — either animalistic creatures without the intelligence to notice, or cyborgs without any emotion at all. Everyone dies.
In H. G. Wells's The Invisible Man, the titular character started out as a psychopath, but it gets much worse after he discovers the ability to turn himself invisible.
Evie Scelan: At the very least, most mages in the novels are highly paranoid. The title character aggressively cultivates a normal life to keep from going crazy herself.
Timothy Zahn's Cobra Trilogy has people being made into Super Soldiers and adjusting to new strength and speed and lasers in a matter of weeks. They're carefully screened beforehand; only the most emotionally stable ones actually become Cobras. Even so, a percent of them do not handle the transition well and develop something called a "Titan complex", the belief that one is so powerful that one is above normal laws and standards. Handing someone all that physical power at once, instead of having to acquire and use it in small increments, essentially sidesteps the usual adjustment mechanisms, according to the books. These people tend to decide that they know what's best and proceed to rebel until other Cobras either kill them or restrain them long enough to have the Super part downgraded. A major plot point is the main character, a Cobra himself, realizing that he has to help his colonies secede from the Dominion of Man and trying not to look like he's developed the complex.
Anthony Burgess' One Hand Clapping is about a man with hyper-photographic memory, who uses this ability to become rich, and show his wife the life she deserves. She later finds out not being able to forget things has driven him insane, when he reveals his plan all along; to show her the good life, before ending both their lives in a suicide pact.
In The Alchymist, one who goes the quick way to being "awakened" will not be able to comprehend the power, resulting in death.
In The Cycle of Fire, the process of mastering fire powers involves experiencing being burnt alive. The trainee must get past the pain to understand the flames, which usually requires sacrificing all capacity for empathy, making them a sociopath.
Michael Swanwick's short story "The Promise of God" is based on the premise that using magic gradually erodes a magician's moral sense until they no longer have any concept of right and wrong; magicians are kept in check by being assigned guardians whom they are trained to obey without question.
Ren Dhark: Inverted in one issue of this German SF series. At one point, the Terran world government decides to start a secret cyborg project with the help of a brilliant and highly ethical scientist. Thanks to very thorough advance evaluation and screening, the actual cyborgs turn out fine and become recurring supporting characters later on; instead, it's a candidate who gets turned down in the end (ironically precisely because he failed one of the psychological test scenarios) who goes a little Ax-Crazy as a result, devises a plan to destroy the entire fledgling institute, and almost succeeds.
In the Keys to the Kingdom series, Arthur begins to slowly get driven insane the more Keys he gets, and almost uses the power of the Keys to kill Denizens several times, but luckily manages to stop himself before he ever does.
Heart of Darkness critiques Britain's "liberal imperialism" in the form of Kurtz who went insane because he has absolute power with no one to stop him.
In Kelley Armstrong's Women of the Otherworld series, this happens to necromancers and clairvoyants over time, and the more powerful they are, the faster their mental state degrades. Interestingly, necromancers can stave off the madness longer by using their powers rather than suppressing them.
Good necromancers are plagued by demanding spirits. They're taught how to erect the mental ramparts but, over time, the cracks begin to show, and the best necromancers almost invariably are driven mad by late middle age. To maintain their sanity for as long as possible they must regularly relieve the pressure by lowering the gate and communicating with the spirit world. ... Clairvoyants also live with constant encroachments on their mental barricades, images and visions of other lives. When they lower the gate, though, it doesn't quite close properly, and gapes a little more each time.note — Kelley Armstrong, Industrial Magic
From Animorphs, we have the Big Bad, Visser Three. In the main books, he's a General Failure with a strategy of brute force that's completely unsuited to the campaign of stealthy infiltration he's waging. Later, the prequel Chronicles books showed the younger Visser as a very capable Manipulative Bastard, exhibiting a thoughtfulness and cunning completely absent from his main incarnation. It becomes quite clear that once Esplin achieved his goal of becoming the only Andalite-Controller in the universe he let the power go to his "head." It was all downhill for him from there.
Certain forms, like ants, can rob one of sentience for a while. It's not always predictable, either.
In gone, every villain (sans Zil) has awesome powers, and most of them are at some point insane from the revelation.
Most notably Penny.
Pocket in the Sea: Lillenthal has shades of this in his character, but it's not clear if this is deliberate obtusification or genuine mental deterioration from living with a telepathic ability.
Seems to be a side effect of becoming a hoshek in The Quest of the Unaligned, as infusing your soul with the fundamental essence of evil is not good for the mind.
A lesser form of this effect seems to occur to Ruahkini. None too level-headed to begin with, becoming the second-most-powerful ruahk in the world infused him with a double portion of wind magic's flightiness and absent-mindedness.
A core premise for Glory in the Thunder. Holding an Aspect greatly taxes mental fortitude, and gods are known for being more likely to go mad the longer they live. This means the immortals in particular tend to have left sanity behind a long time ago.
The Parshendi "forms of power" in The Stormlight Archive involve bonding with a sliver of the Platonic ideal of hatred. Not the best thing for one's mind.
Dexter Quinn is a magnificent example, taking back his own body from possession and using those powers to try to annihilate Earth.
In Vampire Academy, all Moroi spirit users are effected mentally by spirit use, as it is drawn from themselves (as opposed to an element, such as air). And the more they use it, the worse it gets. It manifests itself differently in each individual: Lissa becomes extremely depressed at one point, which causes her cutting, Adrian has bipolar disorder, and Sonya Karp dealt with her insanity by turning Strigoi — although now that she's turned back, she seems to be okay, despite using spirit in most of her free time to find a Strigoi vaccine.
Angel: Season three's "Birthday" depicts an alternate reality in which Cordelia never joined Angel Investigations. Doyle passed his visions on to Angel prior to his Heroic Sacrifice, and Angel retreated into himself in his grief, with the visions only making things worse. Before long, Angel went completely insane from his own loneliness and his visions, to the extent that he would have visions of his victims. The worst of it all is that what Cordelia sees of that version of Angel, a babbling, incoherent mess who starts pounding his head on the wall while talking to her, is, according to that world's Wesley and Gunn, him on a good day.
The 2000 The Invisible Man series starring Vincent Ventresca had the invisibility caused by a synthetic gland that excreted a light-bending substance, a secondary side effect of which (after a long enough period of time) was insanity, until the counteragent was administered - though this was only a plot focus once or twice. The primary side-effects were unpleasant enough that the invisible man usually got the counteragent before the secondary side-effects kicked in. This was due to sabotage on the part of one of the creators of the gland. He intended to use the counteragent to control whoever possessed the gland. Given that they explain Quicksilver Madness as being related to frontal lobe dysfunction, and the frontal lobes are involved in suppressing impulses, the main character presumably wants to be violent but is controlling himself. If they'd done the procedure on someone else, he'd probably have just had Attention Deficit... Ooh, Shiny!.
This trope is applied in Stargate SG-1 to partly explain the evil megalomania of the show's main enemies, the Goa'uld: it's a side effect of revitalizing themselves with their all-healing sarcophagi too many times. When in "Absolute Power" Daniel Jackson asks to be given just a small portion of knowledge from the Goa'uld's ancestral memories, he is quickly shown that that would make him go wonky too. (See also You Are Not Ready.)
In an interesting twist, when O'Neill is exposed to "good" knowledge (twice), he also swiftly suffers mental breakdown; no evil megalomania, but his brain begins to fail from the strain of holding on to it all.
There's also the armband episode, in which O'Neill, Jackson, and Carter get magic jewelry that makes them super-strong and fast. It's rather subverted, though, in that the "craziest" they ever get is kind of impulsive and overconfident.
River Tam from Firefly. See Serenity reference above, as the extent of her abilities is only hinted at until the movie. She's still crazy in both, though.
Spoofed rather effectively on That Mitchell and Webb Look, with a sketch involving a man going insane with his power to levitate...biscuits.
An episode of Farscape had the crew getting their hands on a powerful weapon that attached itself to the user, powered by an addictive drug with all the properties of TV steroids, which they needed to use to get Rygel back. Predictably, D'Argo, Aeryn, and Crichton all had to use it at some point. Thankfully, it had a built-in off switch - if the wearer lost consciousness, the weapon detached itself.
Heroes has Sylar, who went from a bookish watchmaker to a psychopathic serial killer after he began to acquire superpowers. Gaining those powers involves killing people and stealing their brains, so it's kind of a chicken-or-the-egg matter with him.
Season 3 episode 4 shows that his original power was what made him go crazy. When Peter mimics this power, the first thing he does with it (well, the first thing after figuring out how to fix the Sylar watch) is to "figure out" what President Nathan Petrelli is up to... so he cuts open his skull.
And how. As of Season 3 episode 24 (second to last episode of Volume 4) Sylar's latest power acquisition of physical shapeshifting by absorbing other people's DNA combined with psychometry, the ability to touch objects or people and "read" their history by picking up emotions and visions of past scenes has finally driven him completely bonkers, full blown Norman Bates-style crazy. It takes a lot to creep out Sylar, but he finally managed to do it to himself.
The leader of The Company claims that mental illness is a side effect of the mind trying to cope with possessing superpowers, but it's likely he was simply lying to convince Niki to work for him.
Plus, HRG says after they capture Sylar that all the changes to his DNA have made Sylar more and more insane.
Mohinder became more aggressive and developed a compulsion to abduct people and store them in cocoons after injecting himself with his Super Serum.
Subverted pretty well in Season 3 with Scott, the Marine chosen to get the super soldier injection (a variant of the same serum referenced above with Mohinder). After he finishes twitching and panting, Scott glares at his benefactors, demonstrates his new super strength by throwing a chair hard enough to embed it into the wall... then smiles and nonchalantly remarks that he feels good. He spends his brief remaining screen time behaving quite sensibly until Knox sneaks up behind him and 360's his head.
Volume Five Big Bad Samuel Sullivan has been revealed to be this way. His power level is directly proportional to how many evolved humans are present. The more supers are around him, the more powerful he becomes. He's even been described as an ego-maniac who doesn't hold the lives of the normal folk as having any worth, and he'll happily off anyone who stands in his way or hurts any member of his "family."
In the third season of Roswell, Michael the sidekick ends up becoming the back-up king after the real king's (temporary) death and promptly goes crazy and tries to kill his supposedly-destined wife's human husband.
One of the oldest TV examples of this trope is the Star Trek: The Original Series episode "Where No Man Has Gone Before". As Gary Mitchell's god-like psionic powers increase, he becomes a callous megalomaniac, complete with Glowing Eyes of Doom. Dr. Elizabeth Dehner was able to restrain herself long enough to do a Heroic Sacrifice, perhaps because her training as a psychiatrist made her better able to psychoanalyze herself.
In the first series Star Trek: The Next Generation episode "Hide and Q", the focus of the story is Riker's temptation with omnipotence, and how unlimited power takes away his self control and humanity, but fortunately Captain Picard helps him overcome the temptation to save lives and prevent natural disasters. Oh, he uses them to get rid of the space monkeys, so they didn't go to waste.
A more subtle example in "Tin Man": A Betazoid whose telepathic abilities manifested shortly after birth, and are much stronger than normal, instead of at puberty as is normal. He's not crazy, just poorly adjusted and extremely stressed out. Humanoids stress him so badly that he's been hospitalized repeatedly for it. The nature of his telepathic abilities is such that he gets everything about a person immediately, leading to him treasuring the time he got to spend with Data, the only person he ever got to know like a normal person would.
Star Trek: Enterprise. In the Mirror Universe episode, Mirror Archer gets his hands on a Constitution-class heavy cruiser a hundred years in advance of his own. At first he just wants to get credit for its seizure, and end a rebellion against the Terran Empire, but as he realises the power he's got, Mirror Archer becomes more paranoid and power-hungry, eventually deciding to take over the Terran Empire.
Mirror T'Pol: You heard the captain. He's delusional!
Mirror Phlox: It's not a delusion if the captain has the power to do what he says. And from what I've seen he certainly does.
In Millennium, Frank and Laura's unique perceptions of reality lead to mental breakdowns. Frank initially seems to have recovered from his; by the third season, however, he's again fraying at the edges.
In the 2007 remake of the Bionic Woman, Sarah Corvus, Bionic Woman 1.0, goes crazy after getting her power.
A lot of the meteor freaks in Smallville end up going insane and evil. Granted, some of the characters already have a screw (or several) loose before becoming meteor freaks (e.g. Tina Greer, Greg Arkin), but some only went nuts after getting powers. Sean Kelvin for example — before getting powers he was just a jerk, after he got powers he became a Serial Killer. Even the non-killing meteor freaks aren't always all right in the head (e.g. Cyrus Krupp). Also, when normal people get Kryptonian powers, they tend to go nuts (e.g. Jeremiah Holdsclaw, Lana Lang, Eric Summers). Not all meteor freaks and normal-humans-with-Kryptonian-powers go nuts (Chloe Sullivan for the meteor freaks, Jonathan Kent and Lois Lane for the humans-with-Kryptonian-powers), just most of them.
In The Greatest American Hero, Ralph (who lost the manual to his supersuit) meets a filthy rich old man who kept his manual and knew everything the suit could do. The guy used his suit to become rich and crush his enemies like bugs, and the aliens finally took the suit away. The old man thinks it's a good thing Ralph doesn't have the manual. At the end of the story, the old man gives the Lord Acton pagequote and says, "I wonder if he had a suit too."
A major plot arc in Babylon 5 that was mostly abandoned (but still hinted at) with Andrea Thompson's departure was the Psi Corps' attempt to solve the frequent insanity that accompanied telekinetic powers.
In "Journey's End", the Doctor's companion, Donna Noble, somehow gains every bit of knowledge and power that the Doctor has. The Doctor, unfortunately, is forced to give Donna a complete mind wipe of her entire knowledge of the Doctor/the TARDIS/the entire time she was on the show because being the Doctor Donna, as the Ood called her, will kill her. This circumstance means that the Doctor can never see Donna again, as she will remember everything and die. It's not going insane with power that would kill her, it's that humans are physically unequipped to handle a Time Lord mind. Donna was still physically human but with a Time Lord consciousness, and it was going to literally burn her brain out in very short order, certainly before she had time to go mad with power.
A better example would be the Doctor himself. Despite having, essentially, the power of a god, he mostly averts this trope. Except that one time when he was pushed a bit too far, lost it spectacularly, and became, briefly, an example of this trope.
In The Six Million Dollar Man, Steve Austin adapted to his bionic replacements very well, remaining well-integrated and with his reasonably decent moral sense intact. Not everyone else who was given/forced to take bionics did as well. Jaimee Summers was plagued with amnesia and mental troubles, and another bionic man went the "crazy with power" route as well. The implication was that power didn't necessarily go with madness... but it easily could.
In the Buffy the Vampire Slayer episode "Earshot," Buffy is contaminated by a telepathic demon. This has only happened to one other person, and it caused him to go insane and live as a hermit. Buffy is heading the same way when the Scoobies undo the effects.
Willow is apparently the most powerful witch in the world. By a lot. But she's also very vulnerable to going Drunk on the Dark Side, to the point that she once came within seconds of wiping out all life on Earth.
Any human who gains a power. They will be unable to handle it and eventually turn demonically insane.
Also, those who gain Empathy when they weren't supposed to (especially for demons who cannot handle emotion).
The previous Game Master from Infinity Game was driven completely insane by the infinite power he wielded in his world, so he drove everybody else insane and made them kill each other. He eventually grew bored of it and killed everybody.
It's never made quite clear whether the powers of the Awake from Dont Rest Your Head are an outcome of their madness and insomnia or the cause of them, but one thing is for sure: the madder you become, the greater is your power. The more you use your power, the madder you become. It's a slippery slope, and it ends with you turning intoa Nightmare.
Justified in Exalted. After their defeat, the Primordials leveled the Great Curse against the Exalts, making them progressively insane. The madness of the Solars was the canonical reason for the Usurpation and the Sidereals decided to kill the Solars thanks to the Great Prophecy and their own Great Curse. The Solars, as the leaders of the Divine Rebellion against the Primordials, were cursed the most. The Dragon-blooded shock-troopers were cursed the least with the Lunars and Sidereals coming in between.
There's an even more direct example in Elementals. When an Elemental reaches Essence 10, it becomes a Greater Elemental Dragon, an entity of immense destructive power (possibly greater even than the most powerful Demons). All Greater Elemental Dragons to date have been utterly insane, to the point where they've needed to be imprisoned behind some of the most powerful safeguards in Creation (the Gardullis, Greater Dragon of Fire, is imprisoned within the Sun itself). It's speculated that this is because Elementals were simply never designed to be capable of coping with that level of power.
Also justified by Word of God for the Primordials themselves, most of whom are a) kind of crazy and b) have Crippling Overspecialization written into their very beings. They have these traits because for most of their existences, they had been simply too powerful to face consequences to their actions, with even their weak points way outside the power level of anything not prohibited from fighting them. And then the Exalted came into play.
This phrase goes some way to defining Warhammer 40,000. They've even got a Tagline for the game that goes: "Only the insane have the strength to prosper. Only those who prosper can judge what is truly sane." At least some of the Chaos forces admit it - "Sanity is for the weak."
In the fluff backstory of Mage Knight, it was stated that mastering the opposing magics of Necromancy and Elementalism would drive a mage insane. The one guy who did went on to found the Atlantean Empire, which practiced slavery and subjugation.
Cyberpunk 2020 has humanity loss as a side-effect of cybernetic enhancement; as characters become more powerful, they start to feel disconnected from the meatbags around them. Eventually they go crazy, at which point C-SWAT has the job of taking them down.
Aberrant has a "taint" system, which is explained in that "No human being was meant to contain that much power." Taint works in a number of ways. You can purchase a new level in any ability at half price if you take a point of taint with it, and you also take a point of taint when your power reaches a certain level, etc. But no matter how good, or "taint-free" your character is, just remember that this is a prequel to Trinity, where it has already been set in stone that all the Novas went insane.
Not quite. While large numbers of superhumans did go insane, the idea that they all do so is actually Aeon Trinity propaganda and history rewriting.
Call of Cthulhu introduced the Sanity (or SAN) stat. As your characters learn more about the Cthulhu Mythos, their Sanity slowly decreases until they go completely insane. Learning and casting magic also lowers your Sanity, as magic in the Cthulhu setting is a perversion of the natural laws that humans are accustomed to, but then again anything, even mundane stuff like seeing a shadow, can do that in CoC.
Cthulhu Tech, in the grand Lovecraftian traditions, is into this in a big way. Having a chip implanted in your brain so you can pilot the awesome Eldritch AbominationHumongous Mechas drives you mad slowly, being linked to an extradimesional symbiont that makes you essentially into a were-Lovecraftian Beast drives you nuts over time, learning both sorcery and enhancing your paraphysic abilities makes you crazy, and the Zoner parapsysics are normal people who a) got powers by going near a tear in reality that used to be Las Vegas and may be an intrusion into the body of Azathoth, and b) as you guessed, go very, very crazy.
Rifts gives us the Mind Over Matter (M.O.M.) Works, a process that grants incredible strength, reflexes, and Psychic Powers to its users via a set of tiny chips implanted in key spots in their brains. Trouble is, the chips slowly cause mental instability that gets worse and worse with time. The character type that has M.O.M. conversion is called, fittingly, the Crazy.
Vampire: The Requiem subverts the trope at a conceptual level with the new Ventrue, where a Ventrue vampire is more likely to gain derangements and go insane when called to make tests of humanity. And when would they need to make tests of humanity? When expanding their political power. And as the Ventrue are "Lords of The Damned", they would have a tendency to do this a lot....
And in the Revised version of Vampire: The Masquerade, the Malkavians, who have both MalkNet (a sort of hive mind of insanity) but also the super power to make OTHERS share a little insanity.
When the voices are real, are you still insane? But really, the Imbued of Hunter acquired more power the more they threw themselves into the Hunt, thus moving further away from humanity and its trappings (ethics, morals, emotions, etc), eventually turning into super-powered anti-supe fanatics who are human only in biological makeup.
And two of the creeds - the Hermits and Waywards - are broken right from the imbuing, since both get a direct pipeline to the Powers That Be.
The Forgotten Realms setting has Sammaster, one of the mages "promoted" by Mystra to semidivine being status. This impaired his sanity, triggering a delusion (provoked by a ritual) that the goddess was infatuated with him, and the "friendly" advice of an evil priest made it even worse. He ended up stripped of power and convinced that dead dragons shall rule the world due to his bad translation of old prophecy. To fulfill this prophecy, he created dracoliches and the infamous Cult of the Dragon.
BattleTech features several types of neural interface technology that can boost battlefield performance beyond what's possible with just the usual manual/voice controls and neurohelmet, but are correspondingly more invasive and dangerous. Clan ProtoMech pilots, who depend on this kind of interface to control their smaller-than-regular-'Mech war machines in the first place, consequently tend to become increasingly unstable over time and usually die young.
This trope is one of the problems for magick-users in Unknown Armies. Adepts and avatars get their power by virtue of being so utterly obsessed with something (like drinking, or being the ultimate warrior) that it lets them alter reality. This means that most mages are a little nuts by necessity, and need to be pretty committed to their ideals if they want to become more powerful. It's no surprise that one of the big movers-and-shakers in the occult underground doesn't use magic at all.
This is also popular in Warhammer. All magic is made from the powers of Chaos, and chaos likes to reshape things into Eldritch Abominations. If a wizard uses too much juice, the side effects could range from his mind starts coming unhinged, to an explosion with a five mile radius. Being a worshiper of the Gods of Chaos also tends to do this, as their warriors are trying to earn enough glory to become daemon princes, but most end up getting killed or turned into Chaos Spawn long after they go completely insane. Skaven can be described as this, as their leaders tend to have a skewed view on things. Most Dark Elves could also count, especially with Malekeith.
Asyncs in Eclipse Phase are required to take one mental disorder for each level of the Psi trait they acquire because their powers come from a virus written by an alien entity that borders on Eldritch Abomination. And unlike other disorders taken during character creation, they don't get extra character points for them.
Pictured above, Zur the Enchanter from Magic: The Gathering. He was a powerful wizard who went mad trying to achieve immortality and attacked his former homeland Kjeldor. After his invasion was thwarted, he went into hiding and spent the rest of his days aloof from the world basking in his own power.
Whenever anybody attempts to use the Orochi power in The King of Fighters series, it usually results in either death (Rugal Bernstein), insanity (Iori & Leona), or a God complex (Chris, Yashiro, and Shermie).
The Nameless One of Planescape: Torment gained immortality, but at the cost of his memory, which he periodically loses. Each reincarnation develops its own brand new personality, which is often insane. One of them was awesomely so. Another, one of the most dangerous, was mostly sane, but had the little problem of being a complete sociopath.
Final Fantasy VII has its own version; being injected with Jenova cells makes you powerful, but it also leaves you vulnerable to becoming Brainwashed and Crazy -a which is what happened to Sephiroth (though it only started when he learned the truth). And Cloud.
Before Crisis plays the traditional version of it. After summoning Zirconaide, Fuhito's body is transformed as the as-of-yet incomplete summon expresses itself through him. He becomes incredibly powerful, but only retains a bit of whatever sanity he had left (he was kind of a Hojo Jr. already)
Interestingly enough, the only person that doesn't go crazy from Jenova cells is Zack. And we all know how that ended.
In addition, once Cloud is able to overcome the Jenova cells in his body, he never goes crazy again.
Although it is never specified if the Jenova cells caused Hojo to be brainwashed, his injecting himself with Jenova's cells (with an amount that obviously was a much larger amount than that of a SOLDIER) certainly caused Hojo to become a lot more crazy.
To be fair, the Soldier process involved extensive training (I'm assuming there were others around, since it was the Soldier Program — known about by the people living under Midgar, at the very least), and, one would assume, psychological testing, well before any infusion of Mako or Jenova cells. Neither Sephiroth nor Cloud had opportunity for this, since the first was injected before he was friggin' born, and the other was just found as a convenient test subject (and we all know how great Hojo's morals are). Zack is the only on-screen Soldier who was created normally.
Other Soldiers are shown in Crisis Core, and they, too, are normal. (Genesis and Angeal don't count, since they were in their own experiment.)
Why in the first place do you think that there were Soldiers (1st Class at least) who were not created as an experiment at all?
The point is somewhat obscured by Almalexia being just as crazy as Dagoth Ur (in different ways), at least by the time the mask drops and she puts on an evil-looking mask of great power — but the reason theorised in-universe by knowledgeable people and implied by what can be seen is that she went crazy from losing most of her divine powers.
Sotha Sil was a hyper-eccentric shut-in who spent all of his time building a huge clockwork city with a population that consisted entirely of himself, and Vivec was an obscenely narcissistic unrepentant murderer who spoke in nonsensical riddles when he felt like it and thought having sex with the King of Rape was a good idea. Nobody got out of this deal with their sanity totally intact.
In the Oblivion expansion The Shivering Isles, the player character can become the new Sheogorath. This Sheogorath appears in the next game, Skyrim, and is completely mad. This is well justified by the fact that Sheogorath is the god of insanity.
One of the many side-effects of The Taint in Lusternia. Also a result of the Soulless Elixir, which turned many of the Elder Gods' best and brightest into megalomaniacal cannibals.
In BioShock, abuse of the mutagen ADAM, which gives the user incredible powers but often proves addictive, was one of the factors in the downfall of Rapture, the Utopia-gone-wrong in which the game takes place.
Andrew Ryan, on the other hand, didn't need the help to go from visionary leader of an Objectivist utopia to batshit insane dictator. He had no qualms selling Plasmids or ADAM to make a buck, but he didn't use them himself. His crazy was all down to the political power he had, and his desperation to hold on to it at any cost.
In the sequel, Gil Alexander is a good example of this.
Xenogears also features mecha pilots given performance enhancing drugs that cause them to go insane. This has caused the main female protagonist Elly a good deal of trauma, as she's brutally murdered dozens of her fellow soldiers with her bare hands whilst under their influence.
Fei's degeneration into Id, where he unlocks his true power and turns into a lunatic with severe Freudian overtones.
Jeanne D Arc's Liane grows increasingly reckless as she comes to rely more and more on the Paragon's Armlet, both ignoring her friends' suggestions and allowing the Crown to manipulate her. Later on, Roger himself goes insane with bloodlust as the Reaper inside him manifests openly.
Queen Zeal of Chrono Trigger fame went mad with power when she came into contact with Lavos, plotting increasingly dangerous ways to drain its awesome power for personal use, even at the cost of her own kingdom.
In Shadow Hearts, part of the problem with Fusion is that doing so allows the demonic souls the Harmonixer fuses with to attack his sanity. In the first game, this is shown by having to pay a significant amount of the Sanity Meter to fuse. In the second one (and third, though that instead represents Shania losing herself in the power of the spirits), the fused character's Sanity Points run down at a faster rate instead. This is partially mitigated by how Fusion-capable characters start with much more Sanity Points than the rest of the cast (representing the incredible strength of will needed just to use the power).
And in the first Shadow Hearts we find that with great insanity comes great power, as the only way to unlock each Fusion's ultimate attack is to deliberately let your sanity points run out, Guide Dang It.
And Johnny Garland, who has a very, very powerful Awaker form as a manifestation of the Malice that brought him back from the dead, flips out very easily if he uses it, as he doesn't have the benefit of Shania, Yuri, or Kurando's mental discipline.
Giygas, the Big Bad from EarthBound, becomes so powerful in the end of the game that he is unbeatable save for one specific trick. On the other hand, his mind is completely shattered, so he attacks randomly while babbling nonsense.
In the sequel Mother 3, Giygas's "protege" Porky, after gaining the power to travel through time and effective immortality — living for thousands of years — has gone from a mere rotten brat to an insane, murdering Psychopathic Manchild and Evil Overlord bent on destroying everyone in the world but himself.
In The Legend of Zelda: Twilight Princess, several of the bosses Link faces off with have become corrupted by their possession of darkly powerful artifacts. A number of these, such as Darbus the Goron and Yeta the Yeti, were otherwise mild-mannered, friendly characters; the artifacts in question would grant their bearers extreme power and strength, but rob them of their sense and personality. The Triforce itself isn't evil, but it does grant the wishes of those who touch it as a whole, or its parts, whether they are good or evil.
Phazon has this effect in the Metroid Prime trilogy, causing many normally gentle creatures to mutate and go insane. It becomes much more prominent in Prime 3 when it drives the other hunters insane. This turns into a true horror when you see the "Corrupted" version of the Game Over screen in Metroid Prime 3; Samus is overcome by her Phazon corruption and TURNS INTO ANOTHER DARK SAMUS.
SHODAN, who goes insane after her ethical constraints are removed, seeing herself as a goddess destined to inherit the Earth. Officially, SHODAN reexamines her priorities without ethical constraints and draws new conclusions. What were her priorities to begin with? One would assume, keeping the day to day operations of Citadel Station running smoothly. She was also overseeing the station's research projects intended to better humanity (and the corporation's profit margin) through the fields of bioengineering and cybernetics. Which explains a lot of what happened.
From the sequel: "The Polito form is dead, insect. Are you afraid? What is it you fear? The end of your trivial existence? I am SHODAN. When the history of my glory is written, your species shall only be a footnote to my magnificence."
Bionic Commando: Capcom seems to be setting up an implication in the new game in regards to this. However, it is subverted: people with bionic limbs are more or less sane (though in Rearmed, Spencer is one cocky son-of-a-bitch), but if they come to rely on the bionics and have them taken away...they kinda go Ax-Crazy.
The backstory for the new BC game clears this up. The government gives soldiers bionic replacements for limbs lost in battle. Then, the government says that bionic replacements are dangerous and wants to take them back. Which would be fine, except that the people who don't like the idea liken it to the government asking for parts of their bodies. Which is probably justified, especially in the case of people that have bionic eyes. Rather than give up their bionic replacements, these people join a terrorist group.
All in all... wouldn't you get a little annoyed if you were told that you had to forfeit your arm or leg?
In Breath of Fire III, the Kaiser Dragon, if using the Infinity Gene without an attachment of some sort, is uncontrollable and attacks friend and foe alike. The Failure Gene weakens it to the point where it can be controlled. Subverted, however, if you use the Trance and Radiance genes along with Infinity; this creates the true Kaiser Dragon form, which is controllable and stronger than the regular Kaiser.
Somewhat subverted in Xenosaga — Albedo started going mad when he found out not that he was unable to die, but when he found out that other people did die. He began to fear his brothers' deaths and subsequently his being alone for eternity, becoming really morbidly obsessed with death to the point where his greater motivation throughout the course of the series is to find a way to kill himself.
Baron Praxis: The dark eco inside you will eventually kill you, Jak. Its destructive effects cannot be stopped. Once you are in its chaotic grip, it will not let you go until you slide into insanity.
In Suikoden V, the Sun Rune is known to be one of the most powerful runes in existence. Even among the 27 True Runes, its power is extreme, granting both the power to destroy a kingdom overnight, as well as being able to revive a country. However, it also causes mental instability, as the bearer believes themself to be equal to a God, completely infallible, and believing that anyone that disagrees with them should die a very painful death. The King of the ancient Armes Kingdom fell victim to this, destroying his entire kingdom in his insanity, and Queen Arshtat also felt its effects on occasion, and very nearly did the same thing, attempting to destroy her own queendom in a fit of rage and grief after she accidentally killed her husband Ferid due to, again, the effects of the Sun Rune. Falena was only spared this fate because she was slain by Georg Prime, who had promised Ferid he would stop her from doing so if Ferid himself could not.
Supposedly this insanity only results from the Sun Rune being damaged. If one were to also bear the Twilight and Dawn Runes at the same time, then there would be no ill effects. However, one of these runes was stolen before the events of the game, preventing proper use of the Sun Rune.
Grand Maestro Mohs from Tales of the Abyss is a textbook example. He gets glyphs inscribed on him that flood him with Seventh Fonons to obtain the power of a Fon Master, but his body can't handle much of the Seventh Fonon. He immediately turns into a monster, then quickly goes insane. Subverted in that Dist, who applied the glyphs, knew exactly what would happen.
The Malkavian Clan from Vampire: The Masquerade – Bloodlines. While they do gain all the benefit of vampiric powers when Embraced, they also inherit the clan's weakness (which oddly enough, happens to be their greatest power as well) - each Malkavian becomes incurably insane. And if that wasn't frightening enough, they can make you think whatever they want! Yup, their madness power lets them mess with your brain if they want.
In addition to that, Malkavians gain the unique power of insight. This isn't really obvious on your first playthrough, but in the second one you realize that Malks usually know the answer before the question has even been asked. They even seem to know the plot before it unfolds, but they can't make sense out of it. Malkavians have these visions, but they cannot interpret them. They also seem to know the names of complete strangers, but it can be hard to recognize since they tell them in an extremely colourful language and use lots and lots of metaphors. Other vampires, however, can make use of the Malk visions, although it's implied to be extremely difficult to seperate the wisdom from the insane ramblings.
Also, while all Malkavians are a nutjob in some way, the player character is one of the really bad cases. The Malk elder of LA, Alistair Grout, seems to be pretty clear, even if he is suffering from a major paranoia.
The people Malkavians choose to Embrace are always already mentally damaged in some way, so this is also a case of giving insane people great power.
Fire Emblem: The Sacred Stones: To a slight degree, Smug Snake Valter. In the supports between Duessel and Cormag, it's stated that the already unstable Valter once took a powerful magic lance owned by Duessel, when his own lance was broken in combat. The magic of said weapon increased his power, bloodlust, and insanity ever since then.
This is why most of the dragons in Fire Emblem Akaneia made the decision to take on human forms and seal away their true forms within dragonstones. Those who didn't eventually lost their minds and became feral beasts.
In Spiderweb Software's Geneforge games, using the genetic-modification canisters created by the Shapers enables you to build your skills and powers very quickly; but also pushes you strongly into the "arrogant, violent, and insane" realm, which determines the sort of interactions you're able to have with NPCs, and which of the many game endings you'll achieve. In fact, the closest you can get to a Golden Ending (even the best endings are mixed) requires never using a single canister. Using the Geneforge itself guarantees you'll end up a sociopathic Supervillain; most likely a dead one. Some endings are so bad that they border on AnviliciousAuthor Tracts on the corrupting influence of power, and the evils of genetic engineering.
In Psychonauts, those born with psychic potential can develop incredible powers and enter the minds of others. They also tend to range from seriously maladjusted to insane and few are actually all that eager to develop their abilities anyway.
Also, a bit of a subversion, those without psychic powers, when exposed to a material that increases psychic powers, simply go mad, without powers. Also, the only person who's really gone mad with power is Oleander. Everyone else tends to show any issues more clearly because of them, aside from Raz, because of his father's training he subverts this because he has control over them.
In Prototype, the main character is focused and driven, giving little outward sign of anything but rage and determination as he kills and eats his way through thousands upon thousands of soldiers, civilians, and zombies. Normally. But give him a moment to reflect on his situation and what he's doing and...well...
Alex Mercer: The people I've killed... they're in me. I can hear them. See the things they've done. I can understand it all. I'm supposed to do these things...but it's right I can feel it...
Not really an example, once we find out he's not actually Alex Mercer. He's The Virus itself assuming Mercer's memories and appearance. "Insanity" here is really best classified as Loss of Identity, but you can't really call it that when you never even had an original identity to begin with.
Arguably, it's completely inverted, since absorbing all of those people and experiencing their thoughts and memories, including their pain and fear, ends up giving him a conscience.
Pretty much the whole point when playing an evil Cole McGrath in Infamous
Arguably all three of the Brothers Sun in Jade Empire. When you see the Emperor, he is quite clearly out of his mind, being undead and powered only by leeching power from the Water Dragon. "Master" Sun Li seems pretty sane, and has it together enough to pull one spectacularplan on your character. However, he obtains the Water Dragon's power upon his brother's death and drains it even faster than his brother did. By the time you meet up with him for the final Boss Battle, he is very clearly out of his mind.
In Warcraft III, after Arthas claims the legendary (and cursed) sword Frostmourne, it ultimately ends up causing him to do a Face-Heel Turn, kill the king (his father), and willingly join the Undead, even eventually becoming its leader, while quite literally causing him to lose his sanity. Although he had made a few morally questionable decisions to get there in the name of the "greater good", the sword was what ultimately put him over the edge, despite using the same sword to destroy a quite sizable Undead force immediately after obtaining it.
The racial bloodlust of the Orcs which drove Grom and his clan into wild frenzies was a lingering taint in their blood from drinking the demon Mannoroth's blood. As seen in this game, drinking the blood gave the Orcs immense physical strength at the cost of warping their bodies and minds into killing machines.
In World of Warcraft, magic is addictive, and magic addiction makes you insane. And it's hereditary, so the entire High Elf and Blood Elf races are addicts, whether or not they use magic.
There are hints that the Night Elves are also addicted to the magic they use, but nobody really notices because their magic doesn't do things like attract demons, and doesn't seem to have any particular source or concentration.
In the Mist of Pandaria expansion, the Sha are remnants of a slain Old God who feed on the negative emotions of mortals. During the first stage of the expansion, many mortals exposed to it were warped into powerful Tragic Monsters. Garrosh was intrigued by the fact that the Sha power could be used to empower his soldiers, ignoring how many were driven to temporary or permanent madness by the experience. He ultimately stole the heart of the Old God to empower his True Horde, driving many of them, including himself, into a frenzied bloodlust.
In the same expansion, there's also Kanrethad of the Black Harvest — a warlock, who infused himself with a lot of demonic essence. On the plus side, he became a tremendously powerful half-demon. On the minus side, he lost his sanity because of it, and was defeated by another warlock, then permanently banished (kept in a sort of suspended, but probably aware state).
In Portal 2You switch GLaDOS with Wheatley, and the little personality core is now fully in charge of all of Aperture Laboratories' operations... and immediately goes mad with power. Then GLaDOS points out that you did all the work while he did nothing, and he turns against you, sending both you and GLaDOS} hurtling down a pit into the underground ruins of Aperture Science. When the two of you finally get back to the main facility, it's much more dangerous and about to self-destruct thanks to Wheatley's influence. (Doesn't help that he was programmed to make bad decisions...)
In Dragon Age II, it is eventually revealed that Knight-Commander Meredith bought the lyrium doll that drove Bartrand insane. She reforges it into a sword, which explains her increased zealotry and culminates in her animating statues in Kirkwall to fight Hawke and Co.
Malefor, the Big Bad of The Legend of Spyro trilogy, was the first Purple Dragon and had all the power that Spyro can possess. The difference is he didn't know when to stop and let his power consume him, transforming him into a power-hungry monster. His hunger for power was such that it forced his masters to banish him just to protect the dragons from him.
In Starcraft II, the Dominion-loyalist Ghost Nova proclaims that this is what happens to Spectres, an experimental form of "super-Ghost" who have their powers boosted by, among other things, exposure to Terrazine Gas. If the player chooses to ignore Nova's warnings and remain loyal to Tosh, the only free Spectre, and his plans for freeing his captured buddies, they learn that this isn't the case; Spectres are more powerful than Ghosts, but they were scrapped as a Dominion project because the upgrade process automatically blows their Restraining Bolt. Tosh, in fact, not only chooses only volunteers to become new Spectres, but deliberately screens them to ensure they are as sane and stable as is possible for a human with Psychic Powers in the StarCraft universe to be.
In the Modern Warfare series, this is what happened to Vladimir Makarov as the series progressed. Makarov started out as just a soldier in Zakhaev's army, and was promoted to Dragon status, but once Zakhaev died Makarov no longer had anyone holding his leash, and started a campaign of terror thanks to his newfound power. He eventually reached a point where he had virtual control of the entire Russian military, and was attempting to acquire the launch codes for Russia's nuclear arsenal as he wanted Russia to rule all of Europe, "even if it is just ashes."
Torque from The Suffering. Through gameplay, he fills a gauge called the Insanity Meter to transform into a creature that obliterates anything in his path. It's revealed at the end of the first game that the Creature is just Torque hallucinating, and he's actually tearing monsters apart with his bare hands.
In Shin Megami Tensei, it is stated as a very basic law of magic that it is perfectly possibl3 to turn yourself into a nigh-invincible powerhouse by fusing your human essence with that of a demon (or angel, for that matter). Sooner or later, though, you lose all of your humanity. This means you can get your power — but will never be able to exceed a certain point, with the added caveat you've now branded yourself a monster and permanently switched off your conscience. Death almost never fails to ensue. No wonder everyone seems so terribly interested in theHito-Shura, the one case in which the fusion was carried out successfully — and the human ended up in control, meaning the demon power limits do not apply...
In Borderlands 2, Gaige suffers from this heavily when using Anarchy. She even lampshades it by going "I'm going mildly insane!"
In Touhou, RemiliaScarlet locked her sister in the basement for 495 years because she was afraid that this trope would apply. While Flandre is almost certainly crazy, it's uncertain whether it's due to her powers or due to being locked in the basement with next to no social interaction for nearly 500 years.
In Subterranean Animism, there is also Utsuho Reiuji who, well... ask yourself this: If you had been a birdbrained little birdmook who was granted nuclear powers by a Goddess and, over the course of an instant, became Final Boss material, can you honestly say you wouldn't have gone a bit cuckoo and tried to Take Over the World/burn it to the ground?
Rei Ryghts in Hyperdimension Neptunia had this trope when she was the ruler of her nation thousands of years ago after gaining the power of a CPU. Thanks to this, she became a tyrannical ruler who in the end destroyed her nation. She then swore that CPU goddesses should never exist and thus founded the Seven Sages.
Brutal Legend has the Sea of Black Tears. Whomsoever drinks from or bathes in it is granted a portion of Aetulia's wisdom, but also the great sorrow that caused her to cry it in the first place. Though, it's really less "insanity" and more of "soul-crushing emo-ness."
The "Tyranny of King Washington" DLC for Assassin's Creed III shows what would happen if George Washington were to somehow obtain an Apple of Eden. Washington would declare himself King of the United States, turn the Continental Army into the brutal Bluecoats loyal only to him, wipe out whole villages and towns for refusing to bow down, and force the people of New York to build him a giant pyramid palace, while plotting to take the war to Great Britain. The end of the DLC reveals that this was a vision of a possible future given by an Apple that someone has given to Washington. Horrified, Washington has Connor get rid of the artifact and refuses to entertain any thoughts of a monarchy.
In Aquapazza, Ma-ryan and Chizuru are both very powerful and extremely dangerous. Both get better after you've beaten them.
In Mass Effect 3, especially in the Extended Cut and Leviathan DLC, it is heavily implied that the Catalyst is just a (super-powerful) AI that has completely lost its mind, having been created to find a solution to a problem that it has been completely unable to figure out. This goes a long way to explain the Insane Troll Logic that it uses, as well as its absolute refusal to see and acknowledge evidence that contradict its world (galactic?) view.
The Pokémon games have Mewtwo, who is said in various Pokédex entries to have the most savage heart of all Pokémon due to the genetic experiments performed on it to create it as powerful as it is. This is averted in the first movie's depiction of Mewtwo, where his "madness" was more psychological than physical.
A major part of Tsukihime; whenever most characters use more of their potential powers, a direct effect is the deterioration of their sanity. Examples are Akiha's inversion impulse, Arcueid's blood-lust taking over (often called 'Warcueid'), and the protagonist upon using his Mystic Eyes of Death Perception too much. The entire Tohno family has this. In spades.
Arcueid's case is kinda special in that she does not gain additional power during her "blood-lust" mode; she always had that power, "blood-lust" simply makes her no longer hold back. Well, unless she's fighting against Shiki, because only normal women can resist the Nanaya glands.
In Type-Moon's same-universe series Fate/stay night, the Servant class Berserker is definied by the class trait "Mad Enhancement". In exchange for their sanity and access to skills requiring a clear mind, the Servant receives significant boosts to all physical parameters.
In Heaven's Feel, Shirou loses his arm and receives Archer's arm as a replacement. Accessing the knowledge contained within the arm to perform magecraft causes increasing mental damage to him, both psychological and physical.note Parts of his brain actually break under the strain.
Justified in The Order of the Stick: When V has spoken the Four Words, he/she merges mentally with three of the most evil but powerful mages ever, and therefore it's justified his/her mental state is a little vague thereafter.
Then it's later revealed that the splice has as much effect on one's alignment as a cheerleader would on the final score of a game. The fiends just lied to him/her because nothing makes people do a horrible act on their own like having them believe that they're not responsible for their actions, especially when wielding great power.
The superintelligent gerbils of Narbonic, with the exception of the original, Artie. It's explained that, unlike Artie, the other gerbils weren't genetically modified to handle superintelligence, resulting in insanity. But when the sane superintelligent hamsters show up, they're megalomaniacs too. Also, the mad geniuses in the comic are only geniusesbecausethey're mad; if their madness were cured, they would be Brought Down to Normal.
In fact, the geniuses have to go past mad and out the other side... and then it's a crapshoot. Helen Narbonic reins in her crazy because of romantic feelings.
"This is sanity! SANITY!"
Ian Samael of Errant Story starts to fall into this trope after obtaining godlike powers. To his credit, it did take his mother killing herself and his sister to finally drive him off the deep end.
Girl Genius: The Sparks (or mad scientists) are often (although not always) insane. This is explained in the story as a side-effect of the Sparks' realization of their abilities (also known as the Breakthrough), which is usually traumatic to say the least. Some of the insanity seems to be inherent in the Spark itself; even the protagonist, the relatively sane and heroic Agatha Heterodyne, has shown utter singlemindedness and vengeful wrath while in the throes of a particularly Sparky moment.
And also Jack after being sucked into Zimmingham, who's gained always-on ether-vision and the ability to fly while simultaneously losing a large chunk of sanity.
Turns out he was possessed though, so it's something else entirely (although the whole "not eating or sleeping" thing probably wasn't doing anything for him).
The B-Movie Comic: In the invisible killer arc, the process that makes a person invisible can also make them quite frightfully deranged, but only if the person has a basic character flaw to serve as a basis. The scientist then also uses the treatment on Snuka so they can fight the invisible killer, on the assumption that anyone working closely with the professor must be a person of impeccable character. Not quite...
Cwen's Quest: This seems to be case with the Witch Queen. She is easily the most magically inclined character in the series, and while normally smart, she seems incapable of mentioning the word magic without bursting into bouts of insane Maniacal Laughter that would scare most versions of the Joker. It is even more clear she is brilliant but insane in her Twitterings.
In Mark Shallow's current webcomic, Antihero for Hire, Wizard is an example of this, despite being extremely intelligent. Waterfall, an adversary using the same technology, snaps with the same kind of insanity at the sight of her sisters being threatened, which invoked It's Personal.
In El Goonish Shive, this is revealed to be the reason why Immortals "reset" every two hundred years. As time goes by, they become "more bored, more powerful, and less sane," which as Jerry notes is "kind of a bad combo." This could explain why Pandora acts the way she does, since alleviating boredom through certifiably insane plots is her entire reason for doing things. She may have never "reset" in her life.
In Endstone, rocking the Banestone will drive the rocker crazy.
Eerie Cuties arc "Doom Panties". Chloe was always a shy nice girl, except when her friend shared a bottle and "her inhibitions just melted". When her succubus powers got boosted, she began to "feel awesome" and put up quite a show, quickly rolling to the "scary" side. No malice at all, but Chloe won't let anything stand in the way of her fancy — and since she got an excess of power to begin with, for almost anyone else it's a cue to take cover.
The Rage Aspect, especially in the hands of "destructive" classes. Both of the trolls to which Rage powers are bestowed are quite mild-mannered at first. Indeed, half the powers of Prince and Bard involve either the destruction of Rage or the invitation to destroy Rage. It's the other half of those powers, where Kurloz destroys with Rage and Gamzee invites destruction through Rage, that, with the right triggers, make both of them, along with their ancestral counterparts, the Monster Clowns that they are.
And finally, Trickster Mode. Aesthetically, it looks like the polar opposite of Rose's eldritch throes, but it makes a user just as insane, if not more. While it's first engaged by Jane when she licks a lollipop juju laced with a powerful Psycho Serum, it's also a contagious Corruption. It gives any player who engages it manic euphoria, making them appear insanely happy and giving them bright, candy-themed clothing, accessories, and even powers that only barely mask the mental side-effects. The worst part? It spread from Jane to Jake, Roxy, and Dirk before finally wearing off.
Pay Me, Bug!: Telepaths are in danger of going insane if they aren't trained to handle their power early enough.
The Questport Chronicles: The Master of Darkness, one of the most powerful sorcerers in the world, is also utterly insane. It's implied that part of this is due to returning from death.
Orion's Arm, being a civilisation built by Transhumans, has a few interesting thoughts about how radically changing and augmenting one's thought processes could go wrong. Many baselines ("modosophonts") try to boost themselves through the first toposphic singularity to gain vastly more capable (and somewhat alien) modes of thought, but it doesn't always go well...
Ultraconscious Depersonalisation Disorder results from the new mind being a little too introspective, and deciding even its own sense of selfhood is merely a symbol (like the colour red, or the notion of friendship) and view themselves as robots driven by external forces. Sad, but not dangerous to others.
Hyperautistic Sociopathy occurs when the new mind understands the baseline minds of its former peers so well it cannot view them as sentient or capable of independent action, and instead regards them as easily controllable tools or merely animals. The results are seldom pretty.
Trancendence Perversities are dangerously damaged minds that are quite insane, even by the standards of their fellow transapients. Perhaps the transcendee went mad from the revelation due to poor mental fortitude and preparation, or perhaps they devoured the minds of bystanders and developed something a little like schizophrenia combined with multiple personality disorder. And sometimes they feel that eating more minds might sort them out.
Transcendence Blights result when the transapient decides to expand into all the handily available process space around it by becoming The Virus. Sometimes a new transcendee can subsume the minds of those around them effectively by accident, but sometimes they just carry on doing so, probably as a result of something related to Hyperautistic Sociopathy. Some blights have consumed entire planets, planetary systems, and sometimes even spread through space before being beaten back.
And then there's Bloatware, where a mind tries to access vast amounts of data and tooling and secondary systems without actually boosting its intelligence to the point where it can comprehend all the data it suddenly has pouring into its mind. Some just go catatonic, but others can easily turn into Blights or Perversities.
Burnscar from Worm becomes more unstable and violent the more she uses her power.
A lot of characters in Worm fit this trope. Since superpowers can only be gained by going through highly traumatic experiences, most parahumans aren't particularly stable individuals.
This trope is the reason why the world is so dangerous in Brennus. The more powerful the metahuman is, the more likely they are to be insane (and the odds are doubled for mad scientists). Most of the really powerful metahumans are crazy in one way or another, and Desolation In Light, one of most powerful, is an Omnicidal Maniac. Not a good combination for anyone nearby.
Red vs. Blue: The Meta has the abilities and A.I. of every Freelancer it has killed so far. That many A.I. in one body, however, have caused it to be more than slightly snarling mad. To the point where it doesn't seem able to speak itself, only growl. (Church was naturally thrilled when he heard this.)
Church: "Oh great, powerful, and crazy. What a winning combo."
Also in RVB, when Simmons takes power after Sarge's "death" (he wasn't)
Grif: "Simmons, I think you've gone mad with imaginary power."
Simmons: "Oh no, Grif. I've gone mad with very real power."
Raven must suppress her anger, otherwise she takes on a far more evil side that has no mercy and takes up a form that can border on Eldritch Abomination.
Word of God ascribes this trope to the Amazing Mumbo; originally a harmless stage magician, when he got ahold of a magican's wand that was actually magical, it granted him full-fledged sorcerous powers, turning him into a Reality Warper of such power he's even a Domain Holder, with a pocket-plane of which he is the Dimension Lord tucked away inside of his top hat. However, the powers also drove him completely bonkers, and as such he now uses them to create "real" magician tricks that he uses for petty crimes like robbing banks.
Lex Luthor creates a super suit ostensibly to help the police fight crime. The officer testing it builds an unhealthy bond with it and becomes drunk with power, forcing the Man of Steel and John Henry Irons to take him down. Irons later worked out the flaws in the suit that caused this behavior and created his iconic "Steel" armor. Interestingly, the first opponent he fights as Steel is Metallo, the below example.
Luthor poisons unwitting gangster John Corben, then offers to save his life with the Metallo project. Corben, advised only that there may be "some adjustments needed" to help him live a normal life after the process, accepts. But in his new robot body, the hedonistic Corben can't feel, smell, touch, or taste anything, and becomes destructive in his rage at his human sensations being lost. To be fair, Corben was already a criminal and card carrying psychopath.
In Ben 10: Ultimate Alien, this is revealed to be the reason behind Kevin's villainy in the first show. Absorbing energy causes insanity in people with his powers: before he'd even met Ben, he was hooked on electricity and was planning to do things like crash trains full of people together to make a buck. After accidentally absorbing energy from Ben's Omnitrix, he really goes nuts, and turns into a full on Psycho for Hire. After the Time Skip in Alien Force and Ultimate Alien, he is noticeably wary about doing it.
Aggregor claims that this is BS. However, since he is already an insane supervillain, he probably isn't the most reliable source.
Kevin ends up proving it by absorbing Omnitrix energy to stop Aggregor, but loses his sanity and takes the power Aggregor stole in the process. He then proceeds to try to drain energy from anyone with power, right after giving out Disproportionate Retribution to anyone he's had problems with. He is barely stopped long enough to cure him.
Dark Danny, though the whole thing about watching helplessly as your family and friends get blown up and getting your super-powered ghost half removed did give a good start... but he only started a murderous rampage after getting his enemy's power. His past self is extremely horrified.
Vlad, as he seemed to be largely content with his life before the accident. Except that his one true love went and married their idiot friend who Vlad thinks caused the accident anyhow.
In The Batman, when good cop Ethan Bennet becomes Clayface, he goes on a murderous vendetta against his former Captain. It's later asserted that the incident that turned him into Clayface damaged his mind, and that, the more he keeps his form, the less unstable he will be. In fact, whenever he stays in his normal form, he's polite and rational, but the moment he uses his powers, he tends to become violent and unhinged.
In Avatar: The Last Airbender, Aang can enter the Avatar State, channeling the power of all his previous incarnations. You'd think channeling all those former Avatars would make him calm, wise, and experienced, but no... Instead he gets all incandescent and frags everything in sight. However, true mastery of the Avatar State includes being able to control it, which he finally manages to achieve in the Grand Finale.
In the Grand Finale, after being made Fire Lord, Azulaloses it. In an inversion, it's because she's losing power and she knows it — her friends had shown themselves insufficiently scared of her and defied her, and her dad appointed her Fire Lord right before turning the position into "irrelevant figurehead". Paranoia of further betrayals if she didn't instill fear in everyone around her, and insecurity that no one (especially her mother) truly loved her for who she was, caused her to finally snap.
In the case of Doc Ock, it wasn't so much a case of the new powers messing with his brain, but rather stripping away his inhibitions.
John Jameson was infected by alien spores, which made him super massive, super strong, and essentially a Flying Brick without the flying, but, over time, messed with his head, making him filled with rage and aggression. After Venom threw him into a rage (making him think the one messing with him was Spider-Man), Spidey was able to purge the spores from his system, making his body return to normal, but he was severely addicted to the power, and had to be admitted to an insane asylum. The effect was made more evident due to John having a cell right next to the now completely insane Electro, who babbles on about how he has no more human identity.
Eddie Brock would also qualify as an example. When he's stripped of the symbiote in a battle at Peter's high school, he is strapped to a stretcher and removed by two hospital orderlies, screaming at a crowd of spectators that "WE'RE VENOM!" (though he had problems beforehand and the symbiote just released the inhibitions).
Dr. Viper was formally one of two biochemists who invented the Viper mutagen, which was intended to regenerate plants. Then he decided to try and steal it so he could sell it, directly leading to his transformation into the crazed, lizard-like Dr. Viper.
The otherwise peaceful Dr. Greenbox invented Zed, a robot that could repair any mechanical device. When said robot went on a rampage, he initially came along to help stop it...but was so delighted with how powerful his creation was that he tried to sabotage the mission and ended up merging himself with Zed.
An episode of Samurai Jack centers around Jack defeating three shadowy warriors with amazing powers who attack anyone who comes near. After the battle, it turns out that the warriors were actually three men who used a magic well to wish for the power to be the greatest warriors in the land. While the well granted their wish, it also made them blind and took their free wills.
Maria from El Tigre is a mild mannered family woman who is terrified of danger and hyperventilates when it is around her or her family. However, when she puts on her magical glove, she transforms into the 'superheroine' Plata Peligrosa. If she only has it on for an hour, she is fine, but a second longer and she becomes crazed and will do anything for a fight (even free crooks from jail). At one point, she even starts attacking herself (because she has released villains and that is evil) and trying to kill HER OWN SON because he is trying to help her when she has labeled herself evil and attacking herself.
Arguably Manny himself and the El Tigre beforehand. Those who end up with the title of El Tigre along with the belt buckle end up suffering from whether to become good or evil, with the first El Tigre having gone quite mad. However, they also have more raw power and abilities than either the Good or Evil family members. Besides the enhanced capabilities along with chain claw, the first El Tigre explains that Manny will keep getting new powers after demonstrating some such as growing the claws to be quite long and a sonic explosion-causing roar. Lastly, El Tigre has the power to escape the underworld with the power of the Ancient Tiger Spirit.
Rampage. A Maximal experiment to create an immortal spark, he is nigh-immortal but also completely insane and takes great pleasure from torturing others in the sickest ways possible. He's also a cannibal.
Megatron too. Once he mingles his spark with that of his namesake, and then takes control of the Nemesis, he goes completely bonkers and begins quoting the Transformer bible.
Optimus Primal had a couple of instances too. When he carried the spark of Optimus Prime, he not only got a new body out of the deal, but took on some of Prime's mannerisms. Prime was an incredibly good character, so Primal didn't get the nasty side effects Megatron got. In another episode, he gets injected with a serum that was supposed to turn him into a coward. However, it instead removed all fear, turning him into an unstoppable berserker. He didn't get stronger, just fully utilized his already considerable strength.
In Static Shock, the Big Bang was occasionally thought to invoke this in earlier episodes. Notably, it's why Richie refused to trust Static when the metahuman Replay was framing him, believing that Static just took longer to go nuts than the others. However, since the Big Bang took place in the middle of a gang war, the guys who got the highest doses were generally not great people to begin with, and later episodes introduce other perfectly sane superpowered characters.
Inverted and then played straight in ReBoot. Hexadecimal started out very powerful and insane. When she gets reformatted into a sprite and as a result is depowered, she becomes very sane and cheerful. But then she needs to go viral again to fight Daemon, and the powerup makes her insane again.
In the season 5 two-part opener, Farmworld Finn wears the crown and starts going insane too, complete with maniacal laughter.
Subverted, however, by Flame Princess in "Vault of Bones". As she is attacking the monsters with her flames, she shifts into One-Winged Angel form, laughs maniacally, and generally seems to be going this way... but then, when one of them grabs Finn, she opts to intimidate it into letting him go, rather than risk hurting him by burning it, and she seems perfectly normal later.
The EVOs of Generator Rex can get hit with this, Body Horror, or both. The clearest example would probably be Breach, though she's at least coherent. No-Face from the Bug Jar also demonstrates a seriously degraded mental state, though not in the same way as Breach. And some EVOS are so far gone, it's easy to forget they were ever human in the first place.
Nerissa of W.I.T.C.H. is a sad example of this. She was actually pretty well-adjusted until she gained control of the Heart of Candracar. The Oracle tried to cut her off at the pass, making her hand it over to her best friend, Cassidy. All it did was drive Nerissa farther into needing it, in which she killed her friend in cold blood. A good generation later, she comes back, pulling off a plan to obtain more Hearts "for the greater good of the universe".
In a less malevolent example, an episode of Aladdin: The Series had Genie's powers be transferred to Iago. Iago notices pretty quickly that, along with Genie's powers, he has also become more eccentric and strange, while the de-powered Genie becomes more morose. Apparently, possessing semi-phenomenal, nearly-cosmic power makes you a Cloudcuckoolander.
In "Magic Duel", Trixie's sanity deteriorates while under the influence of a magic-boosting Artifact of Doom; the first thing she does after banishing Twilight being to turn Ponyville into a micronation with her as its Caligula, then it comes to a head when she has Snips and Sails drag her chariot across the ground, as she is now so paranoid that she can't trust wheels.
In the X-Men animated series, Apocalypse was this according to Professor Xavier, which means that unlike Magneto and the group behind the sentinels, he cannot be reasoned with and has to be stopped at once by the X-Men before he will destroy the world.
In the Batman Beyond episode "Heroes", three scientists are transformed by a Freak Lab Accident. They are angry and frustrated at being unable to lead normal lives, and are pushed over the edge when they learn that the transformation is killing them and driving them insane — and that the "accident" was deliberately set up by a colleague who had intended to kill his romantic rival.
Sugilite from Steven Universe. A building sized fusion of Garnet and Amethyst, she's a walking wrecking machine. Problem is, the combo of Amethyst's wild personality and Garnet's power makes her violent and hard to manage. If the fusion lasts too long, she becomes a serious threat.
Adolf Hitler. Though potentially an inversion, in that he may have gained his power from his insanity. Memoirs of many German officials close to Hitler portray him as becoming increasingly, even violently, erratic; particularly toward the end of the war. Historians tend to attribute this to an increasingly self-destructive drug addiction. Amphetamine use was common in the German military at the time, and medical records show that Hitler himself was prescribed amphetamines in increasingly large doses, combined with barbituates later on to counter the side effects. Close to the end of the war, pilots were supplied with a formidable drug cocktail containing several analogs of amphetamine, cocaine, and morphine. The cocktail was quickly made available to submarine crews, and certain front-line combat units. It's believed that Hitler himself may have been using this particular combination drug by the end of the war, which contributed to his severe irrationality and eventual suicide.
There are also indications that Hitler had some type of neurodegenerative disease at the time of his death. Some such diseases cause these same symptoms. In combination with the drugs he may have been taking, and the distinct possibility he was insane to begin with, the results could be messy.
The Roman Emperor Caligula, who made his horse a consul (the Roman equivalent of Chancellor) and collected sea shells to prove he owned the sea. More accurately, he claimed at a dinner party (full of hostile nobles) that he could choose to make his favourite racing horse a Consul if he wished. The sea shell thing was an aborted attempt to invade Britain, with him ordering the soldiers to gather shells and pebbles from the shore either so he could ride home through a Triumph because he conquered the ocean or to shame them for refusing to cross the Channel. There is a relatively small but persuasive claim that he wasn't really that mad, he just had a lot of unpopular ideas and an unusual sense of humour. The few accounts from the ancient world about Caligula were very much hostile to his rule and they don't entirely match up with other evidence from his reign (his coinage wasn't unusually debased, meaning that if he was mad he managed to be so cheaply for the state).
At least those rumors about the cause of Catherine the Great's death were just an urban legend.
Some studies have shown that those with higher IQs are more prone to mental illness.
Many serial killers are highly intelligent people. For example, Edmund Kemper has an IQ of 136 and Andrey Chikatilo was a very educated man and an avid reader (It's quite possible that he was only staging schizophrenic-like madness to avoid a death sentence, as it looks quite overacted).
It can reasonably be argued that serial killers being more likely to be highly intelligent may be a simple selection effect. To avoid being caught for an extended period while killing people would require substantial intelligence. Stupid people who might otherwise be serial killers are more apt to be caught simply because they won't be so able to cover their tracks.
It is known for a certainty that at least some forms of schizophrenia result from abnormal connections being formed between different portions of the brain. We also have strong indications that about 50000 years ago our species underwent a fairly rapid mental change that resulted in a veritable explosion of culture. Further, the kind of integrated intelligence humans posses, with such abilities as being able to intellectualize about our own mental states, is known to be absent in other apes. It is quite likely that schizophrenia is part of the penalty we pay as a species to have the kind of lethally effective intelligence that has made us the deadliest animals to ever exist on this planet. And it sucks beyond the ability of a non-sufferer to understand to have schizophrenia.