As it eventually turns out, The Engineer from Amnesia: A Machine for Pigs is a cackling megalomaniac who wants to exterminate all of humanity because it's a fragment of the protagonist's consciousness given life and driven completely insane by the atrocities of the 20th Century.
Amnesia: Memories has Ukyo's darker, evil side. This side of him seems to pop up out of the blue and his facialexpression makes it clear that whatever is keeping his last shred of sanity around is hanging on a silken thread. He makes no secret of his vast, but creepy knowledge about the heroine and the people around her, and often gives thinly-veiled lines to get the heroine into danger. He delights in telling her that he'll kill her and he succeeds in killing her in various Bad Endings.
Anyone who has been injected with some form of the Lazarus Vector: Lazarus Elixir, Lazarus Phoenix Pinion, Venom, or Titan. Who made these potions and how they are interconnected in the series is a major spoiler, but is responsible for about half of the plot. And yes, Batman also took it.
The Penguin is not as obvious as The Joker, but he is extremely sadistic, bloodthirsty, and cruel. His methods of torture are pretty barbaric.
In a strange way, the player character makes Batman crazy. What else would you call gliding around Gotham City and Arkham Asylum looking for gang members to horrendously punch, explode, electrocute, freeze, glue, hang, and humiliate, all while leaving them alive and in tremendous pain/fear? For "EXP" so that you can equip Waynetech that Batman already has? OR leaving criminals to do their thing while you scour the map looking for trophies that look like green hooks?
Lilarcor the talking sword in Baldur's Gate II. It's a fairly powerful sword that houses the soul of a bloodthirsty idiot. Every bit of Lilarcor's dialogue consists of it whining about how it's not killing anyone at the moment and asking you to hurry up and kill someone with it.
Bendy and the Ink Machine: By the time Susie Campbell gets turned into an ink creature, she is largely Lost in Character and has also developed multiple personalities, one of which claims of her murders that Gollum Made Me Do It. She has personally eviscerated at least ten other Toons, whose corpses she leaves on display. When Henry goes to see her on Level 9, she's in the middle of torturing a Butcher Gang clone, and between Chapters 3 and 4, she kidnaps Henry's friend Boris and reforges him into a minion. She suffers from mood swings and sometimes sets her own outbursts off with her monologing, such as the time at the end of Chapter 3 when she breaks down in the middle of talking about heaven - first crying then laughing - and sabotages the elevator with Henry and Boris on it. Other things set her off too, like the time at the end of Chapter 4 when she enters into a murderous rage because Henry survived the Boss Fight with Boris.
BioForge: The victims of the cyborgization process kept becoming this until the Mad Scientist in charge found out how to stop this. (The player also has the option of acting Ax-Crazy themselves, as there are points where you can kill someone even though they pose no threat to you.)
The Splicers in BioShock have been reduced to this from repeated use of the Psycho Serum ADAM. Later you learn that they've been targeting you specifically because the ADAM they've been using has an additive that makes them susceptible to Mind Control by Andrew Ryan. If it weren't for that, they'd probably going ape on what's left of each other as well. By comparison, the Splicers in the sequel seem to be in better control of their faculties, if still mentally unstable and in the habit of constantly talking to themselves; the Spider Splicers may be an exception.
In a vein rather similar to Kefka, there's Yuuki Terumi. He trapped the world in 725 time loops of 100 years each through which the world, during just the first 10 years, got ravaged by an enormous, hydra-like monster. This is also arguably one of the less horrifying acts he has done, solely because the result of it was a continuously repeated impersonal and indiscriminate mass-slaughter on over 50% of the human race... Y'see, it's not until Terumi gets personal with people that his maniacal depravity really shines through. He gets worse after reuniting with his original body, the Susanoo Unit. He becomes a lot more serious and focused and actually has the power to back up his threats.
Carl Clover. Especially if you touch Nirvana/Ada, or try to hide information about his father Relius. In Tsubaki's story mode, losing to him leads to him killing his childhood friend.
Carl: Oh, please don't die yet, Miss Tsubaki... Tsubaki: ... U-Ugh... Carl: Well, if you'd be so kind as to tell me the whereabouts of my father... Preferably before you bleed to death.
With names like The Mad Dog, Massacre King, The Genocider, and The Deathbringer, it's extremely clear that Azrael's an extremely dangerous being who relishes considerably too much in the carnage, so much so that he had to be locked inside a dimensional prison to prevent him from destroying anything and everything in sight.
Act 3 of Central Fiction reveals that the kind, loyal, hardworking Hibiki Kohaku we saw in Chronophantasma was a mask that hides a deeply disturbed individual. He's willing to kill Kagura to become truly emotionless.
Krieg, the playable Psycho, deserves special mention for taking this trope literally, with his special skill being to run around with a giant axe.
And those are just the most obvious ones. *MOST* of the cast (first, second or one-and-half one... OK, may be inhabitants of Elpis are a bit more sane) is ax-crazy. And those that are not, still see murder as a pretty normal thing. This is this kind of game, yes.
The Sheriff of Lynchwood, AKA Nisha Kadam, in Borderlands: The Pre-Sequel! and Borderlands 2. A Trigger Happy sadist who loves watching her enemies die, her "retirement" from Vault Hunting consisted of ruling a town with an iron fist, and filling it with bandits just so she had something to kill.
Fritz from Brain Dead 13, whose deranged methods of killing involve eggbeaters, harpoons, and a blender. Combine that with him being a Determinator, and, well... you're screwed.
Sunshine Bear, the mascot-costumed serial killer from Camp Sunshine, is a psychotic maniac who butchers everyone in his path indiscriminately, leaving a trail of bloody corpses littering the game's entire layout.
Clive Barker's Undying: Most of the Covenant family are insane in one way or another, like with Ambrose beating his father to death with a pool cue simply because he was tired with him meddling in personal affairs, to Bethany settling a rivalry with her brother by gleefully removing his jaw, tying him up in the basement and setting a swarm of hungry rats on him.
Dark Souls has Lautrec, who is a loner vagabond who uses and kills Anastacia, the fire keeper of Firelink Shrine. The Darkwraiths you meet in-game became Ax-Crazy after becoming Drunk with Power and undead that have lost all their remaining humanity are insane, violent shells of who they once were. That said, some hollows you find are docile and often huddle themselves into corners, crying.
The Brotherhood of Blood in Dark Souls II are murderous wingnuts who worship the God of Blood. Their leader, Titchy Gren, gives a huge rant about slaughter when you meet him.
Mound-Makers in Dark Souls III are easily the least stable of the "evil" covenants: Rosaria's Fingers and the Aldrich Faithful have religious or personal motivations, but the Mound-Makers exist solely to rampage around killing anyone in their path and are rewarded in the same way for killing fellow invaders as they are the Host of Embers. The Mound-Maker NPC you can encounter, Holy Knight Hodrick, views the skulls of his victims as "family"; when you summon him for a rematch on the Road of Sacrifices, instead of focusing primarily on the player like most invaders, he'll charge blindly at anything that crosses his path...which can save you a lot of trouble, if you kite him towards Yellowfinger Heysel or the Exile Watchdogs since those won't respawn if he kills them.
Arguably the most disturbing example is Brandon Whittaker of the second game. A C.U.R.E protester who watched people die during the outbreak and subsequently lost his mind. He now serves "the cause" by deliberately feeding people to zombies to turn them. When Chuck protests, he goes absolutely batshit and attacks him with a giant glass shard. What's worse, after you defeat him, he gets bitten by the same zombie he tried to use on a hostage, and immediately commits suicide.
And, if Off the Record is any indication, he is the one who started the outbreak.
Crypto from Destroy All Humans!. He's a nigh-unstoppable killing machine who plans to rip out your brain and destroy your entire town.
The Darkspawn of Dragon Age are a race of Ax-Crazy Blood Knights. Being the embodiment of The Maker's condemnation of mortals (according to The Chantry), the Darkspawn exist to kill and do horrible things to the world. They will do their best to kill anything and everything they come across that isn't also Tainted; if they can't find anything nontainted nearby to kill they will readily try to kill each other. Only an Archdemon has the power and will to unite the murderous bastards and keep them from killing each other when they get bored. Unfortunately, Archdemons, being Tainted dragon gods, are also Ax-Crazy. That's why Blights happen. Even some of the "freed" Darkspawn in Awakening are Ax-Crazy.
In Dwarf Fortress, every now and then one of your dwarfs may snap and start killing anything and everything in sight. The usual cause for this is their happiness falling too low, although there are others. Often from having seen/being attacked by another dwarf that had this happen to them.
Memorably, in the Something Awful Let's Play fortress of Boatmurdered, the fortress deteriorated to a level where it was in a constant state of anarchy, with dwarfs becoming so annoyed by the smoke and miasma that they snapped and went on a rampage, invariably killing a dwarf that was the relative of another dwarf, driving that dwarf into a homicidal rage, and so on. Probably the best individual moment of this was when one of the former governors of the fortress, who was possessing of insane strength, managed to beat several other dwarfs to death incredibly messily. DID I MENTION HE IS ON FUCKING FIRE!??
A (rather appropriate) meta example from that very Let's Play comes in the form of StarkRavingMad, one of the appointed overseers of the fortress, who devises a plan to finally end the siege of elephants that was threatening to kill them all:
StarkRavingMad: I've started project 'Fuck The World,' a top-secret attempt to funnel magma to the outside. I'll kill those elephants. I'll kill all those fucking elephants.
Sheogorath, the Daedric Price of Madness, oddly enough plays with this trope. For someone who ends all his conversations with "Visit again or I'll pluck out your eyes!", he is surprisingly nonviolent. Even if you attack him, he doesn't attack back directly, just scolds you and teleports you into the sky, causing you to fall to your death. (Or possibly leaves you where you are, and hurls the planet at you. He's used a Colony Drop before in the Backstory after all...) Just don't make him too happy or talk to him while he's contemplating having Brain Pie for lunch. Further, he's actually not technically in control of "Axe Crazy" only but Madness as a whole. Mehrunes Dagon and or maybe Boethiah might be more "Axe Crazy" since they're both a lot more violent and associated with violence. Dagon in particular likes to sent you on a mission to kill people who he dislikes, if he's not trying to smash you himself.
The infamous Therana from Morrowind is a Really 700 Years Old wizard whose life-extending necromancy doesn't seem to have worked so well on her mind as the rest of her fellow Telvanni. She is at all times incomprehensibly senile, which wouldn't be so bad if not for her habit of turning murderously violent on a whim.
In the Tribunal expansion, Tribunal deity Almalexia proves herself to be this. While she appears to be fairly stable initially, the one-on-one confrontation with her at the end of the expansion reveals just how far gone she really is. And the ensuing battle shows just how dangerous she is.
Glarthir, from Oblivion. He believes three of the other NPCs are conspiring against him and sets you to tail them. Not a single one of them actually is — but if you tell him this, he'll assume they've turned you as well, and whip out an axe to take you out before you get him. If you lie, he'll ask you to kill whoever you fingered, or do it himself if you refuse.
Nikolai Diavolo in Everything or Nothing. Though he never shows this trait within the game, M reveals that even the KGB tried to kill him. To make matters worse, his mentor was the VERY ax-crazy Max Zorin.
Kefka from Final Fantasy VI. This is a man who reduced the planet to a burned-out shell because he could, and then randomly took out the remaining pockets of civilization with a Wave-Motion Gun whenever he got bored. That's during his encounter with the Warring Triad. Beforehand, he was, while relatively normal, a man prone to breaking into fits of hysterical laughter when doing atrocious crimes, had brainwashed Terra and ordered her to torch fifty of his own soldiers alive, attempted to burn Figaro, poisoned Doma (and in that particular case, he also mused maniacally about his joy about listening to "music" revolving around hundreds of people screaming in unison), brutalized various Espers in the Magitek research facility while maniacally declaring himself to be a god, massacring various Espers at Thamasa as well as burning Thamasa to the ground.
His backstory reveals that he was the first Magitek Knight, but the process was imperfect, and that he became insane after the process, which is heavily implied to involve brain surgery, meaning that the Empire broke his brain.
Jerme, the Black Fang assassin nicknamed Death Kite from the same game, fits this one like a glove. He says to Lyn: "You! Woman! You have beautiful skin. If I sliced you into lovely red ribbons with this, would the pieces be as soft and delicate as silk?"
In a non-villainous example, Peri takes a huge liking towards murdering people with little provocation.
One of the villains, Hans, a criminal who is also one of Garon's subordinates, plays this trope straighter. He literally yells with joy as he cuts people down. Bonus points for using an axe.
The Conquest route has Takumi, who starts the game out fairly sane, if cynical. However, over the course of the game, a combination of you betraying him, his lingering self-esteem issues, and a heap of Demonic Posession slowly chip away at his sanity. Getting thrown off the Great Wall of Suzanoh is the last straw, and when you see him for the next and last time, he's a maniac who is completely obsessed with killing you and razing Nohr to the ground, and all you can do for him is put him down like a rabid dog.
Dimitri starts off burying this side of him deep down inside, but unless you choose the Crimson Flower route, the Wham Episode that is Chapter 11 and the five-year timeskip turn him into a man completely obsessed with killing Edelgard and perfectly willing to brutally slaughter anyone who gets in his way. On the Azure Moon route, he gets better; not so much on Verdant Wind or Silver Snow, where his newfound recklessness gets him killed. On Crimson Flower, he's much more composed, but has blinded himself to the craziness of the person down below.
Rhea/Seiros becomes this on the Crimson Flower route. After Byleth sides with Edelgard in Chapter 11, she wants nothing more to kill Byleth and gleefully screams about how she wants to rip their heart out. By the end of the game, she is willing to burn all of Fhirdiad to the ground in order to exact her revenge (despite condemning many innocents to death in the process).
General Barclyss is a psychotic nihilist with a penchant for bombarding defenseless villages and has the goal of destroying all life on the planet. While he tries to keep up a stoic demeanor during his atrocities, he's also easily provoked by setbacks to the point of slashing away at any living thing (or non-living, as shown by Palisade Prison's hallways). By the end of the game, he no longer hides his unstable nature, as shown by his insane ranting and raving about the meaning of life.
His second-in-command, General Kail, is so addicted to killing that he treats any period of time when he's not killing as equivalent to holding his bladder. When Barclyss gives him permission to kill volunteers for their army, Kail remarks that it's been far too long since he last did anything like that. This is especially noticeable when he states he can't hold back on firing the Aquadome's cannon at Heyama a second time, despite Barclyss being knocked unconscious and unable to give him orders.
Catalina. In III, she already shoots two of her apparent love interests, but it goes even further in San Andreas: she has graves dug by her house, she sexually abuses CJ (and still thinks she loves him), she threatens to kill/castrate CJ, and she takes glee in killing any cops or civilians who get in her way.
T-Bone from Grand Theft Auto: San Andreas. He is brutally sadistic and impulsive when gets really angry. In "The Introduction" video, he gives a brutal beating of another gang member, possibly to the point of killing him, and at one point threatens to rape and kill CJ's family as a "joke."
Tommy Vercetti from Grand Theft Auto: Vice City, is aggressive and easily angered, and has no problem killing his victims with chainsaws (not for nothing he was nicknamed "The Butcher of Hardwood"). Also one occasion he was sent by the leader of the bikers to wreak havoc in the city.
All is not well in Niko Bellic's head. This is probably the result of PTSD and grief. He has also a deep talent for carnage and always fought like a raging berserker. What would you call someone who massacres dozens of gangsters for a few dollars, all while he curses and screams, and commits dozens of acts of vigilantism for the same reason?
Niko:[after killing two random gangsters] AAAAAAAAAAARRGH MOTHERFUCKERS!!!
Ray Bulgarin. He's extremely impulsive, violent, aggressive, and paranoid, to the point of shooting his own sister unprovoked, possibly killing her. Later, he beheads the cook who stole his diamonds and presents the disembodied head as a death threat to Luis.
Trevor Philips is, to say the least, completely fucking insane. The trailer alone showed him brutalizing random people and committing wanton destruction for kicks, but that merely scratches the surface: he's completely unpredictable, extremely violent, and disturbingly easy to piss off, he's a meth addict and an alcoholic, he is heavily implied to occasionally eat people.
From the same game, we have Steve Haines, a showboating (literally — he has his own TV show) FIB agent who's just a little too eager to bring the protagonists in, Peter Dreyfuss, a murderous film director who was behind the disappearance of a starlet named Leonora Johnson in the '70s, and The Altruists, a cult comprised of former baby boomers who take in "lost souls" to "purify" them.
Michael DeSanta himself. He's just very good at hiding it. Split Personality and anger issues aside, he sounds pretty psychotic during random killings.
Michael: [during random killings] AIN'T THE AMERICAN DREAM GRAND!
Kilgor from Heroes of Might and Magic III is Ax-Crazy even by the standards of the Barbarians. The bloodshed and destruction wrought during his campaign to become Krewlod's new king left Krewlod in a state of near ruin and starvation. His narration during the "Festival of Life" campaign and the thoughts of other characters depict him as a madman who relished slaughter. His madness ushers at the end of Enroth when he acquires the Sword of Frost and wages war on the rest of the world. Gauldoth Half-dead from Heroes of Might and Magic IV muses that the Reckoning (triggered by the Sword of Frost and Armageddon's Blade clashing) arguably did just as much good as evil since in addition to destroying Enroth it also ended Kilgor's life. In Gauldoth's opinion, Kilgor was so awful that losing the planet was an acceptable price for ending him. The Immortal Hero Tarnum blames himself for Kilgor's rise to power since his own brutal stint as the Barbarian King set a bad example for Krewlod to follow. Because of him, the people of Krewlod considered Ax Craziness a positive trait.
The Prophet of Truth, who in Halo 3 rants and raves and comes close to his goal of activating the seven rings that will kill all life in the Milky Way. While his portrayal in the novels and Halo 2 is more that of a cunning and sane manipulator, he was apparently driven to madness after the events of 2, when 343 Guilty Spark revealed to him the full truth about the Halo Array's purpose (though he already had his doubts to begin with).
Ripa 'Moramee, the Arbiter from Halo Wars, is an even clearer example; he's loyal to Regret, but otherwise cares only about killing things.
Daniella the maid from Haunting Ground. She is a homunculus created to be the "perfect woman" by her master. However, she cannot feel pain or pleasure, which drives her to scream at her reflection because she feels she is not "complete". She becomes even crazier and hateful towards Fiona, who can feel what Daniella cannot. Daniella comes to a logical conclusion - she must obtain the vital essence known as Azoth, which Fiona possesses, in order to become complete. She plans on tearing the Azoth right out of Fiona and pursues her relentlessly throughout part of the game. She is prone to fits of maniacal laughter (her head often spasms violently when she does so) and will shriek at random. She also cries blood and licks a glass shard to complete the effect.
Supplementary material overlaps this with Break the Cutie/Broken Bird - Daniella is actually a human, but was kidnapped by her master as a child and subsequently drugged and tortured to the point that she lost both her memories and the ability to feel pain or passion. In addition to this, a lifetime of monotonous maid tasks in a castle with only three other occupants (at least one of whom is shown to abuse her), her own infertility, and her belief that she's some kind of android drive her mad through her own numbed existence. Looking at her backstory, it's pretty evident that her jealousy towards Fiona is just what pushes her over the edge.
Lt. Carter Blake in Heavy Rain is a Rabid Cop who seems unfamiliar with the concept of restraint when performing his duties.
Jade Empire's Black Whirlwind is a hot-headed hulking brute who carries twin axes, has killed innocent noblewomen, burned down temples, and more, most if not all while drunk, but he's on your side.
Captain Hayashi from Jet Set Radio Future will attempt to murder you for your petty crimes, sending in all the tanks and using an experimental Spider Tank.
Larxene show this trait throughout her life as a nobody. She often mocks and ridicules allies as well as enemies for a fake feeling of enjoyment, and seemingly enjoys inflicting physical as well as emotional pain on others including Sora, Namine, Roxas, and even Elsa.
Axel displays this trait much. Like Larxene, he pretended to enjoy hurting others and also flashed a wide grin at such moments. He carried out a series of betrayals throughout the series and killed several of his allies ruthlessly for his own motives and sometimes just for his personal enjoyment.
Roxas also shows this trait after realizing the truth of his existence, attacking both of best friends ruthlessly and killing one while injuring the other, in the process. He also recklessly destroys the central computer of Virtual Twilight town and keeps attacking DiZ's hologram in spite of knowing that he could not attack DiZ in real. He even goes to the extent of attacking his own somebody, Sora.
In the sequel, HK-47 is back again, and it's possible to pick up a truly psychotic Wookie named Hanharr. Atton is also implied to have more than a little of this. Much more than a little, if you encourage it...
There are a slightly worrying number of characters exhibiting these traits in League of Legends, ranging from demonic swords to two different Mad Bombers to artistically inclined serial killers to murderous wolf-men, but one of the craziest has to be Kled. A tiny, ornery Yordle who serves as a sort of mascot to Noxian soldiers, Kled's philosophy of life is pretty simple: Kill everyone who trespasses on his land. Also, the entire world and possibly some other stuff too is "his land". Luckily for Kled, he really enjoys killing people, plus the voices in his head keep suggesting it, plus there's good eatin' for either him or his pet lizard on some of those enemy champions, so he is absolutely fine with it.
Majora from Majora's Mask. Unlike most of the other villains in the franchise, its main motivation for killing/tormenting/etc. its victims is for sheer fun and tries to take it to its logical extreme by crashing the Moon into Termina.
Zant from Twilight Princess, as shown with him killing the queen of the Zoras, enslaving the Twili against their will, and his unnerving confrontation with Link and Midna at Lake Hylia, where he tries to kill the latter. By the end of the game, the craziness fully manifests itself as he's reduced to a screaming, maniacal wreck.
From The Mark of Kri we have Tati. This deserves special mention because her older brother Rau is the one with the ax, but he's remarkably calm and level headed. And he never speaks in anything except battle cries. Unless Tati turns to The Dark Side, in which he will utter her name once before Tati and the forces of darkness slaughter him. Tati, on the other hand, is very vocal and has moments of exceeding maliciousness. The way she slaughters single enemies with her knives looks more painful than anything Rau does with his ax, and considering the body count Rau racks up with it, that's saying something.
In the Mary Skelterfranchise, any Blood Maidens that fall into Blood Skelter start qualifying for this and then some on the spot. It doesn't matter how they are normally: once they fall into the state, they immediately become raving berserkers that can't even communicate with others outside of Laughing Mad and the occasional pained screaming. Yes, that includes what are normally the absolute nicest ones, so poor Sleeping Beauty doesn't escape this possibility at all.
Hell, as Mary Skelter 2 revealed, even resident Nice Guy Jack isn't immune to falling into this trope because guess what? He has his own pseudo-Blood Skelter known as "Ripper Jack" (yes, the Jack the Ripper parallel is for a damn good reason), which causes him to go from illegally nice Non-Action Guy status to beating down on both enemies and allies with an extremely unnerving Slasher Smile decorating his face. Even if we only see the smile on his Marchen mimic.
Also in Mary Skelter 2, we have the Mysterious Nightmare, a monster that is only capable of thinking of killing everything in sight... with the exception of Jack.
In the first MechWarrior, you can get pilots to help you in your group, each comes with a picture and a quote along with their stats before you hire them. One of them is Killer, wearing a ski mask, with the quote "I want to burn villages and eat dead, burnt bodies, kill, kill, kill!"
The quote is from the 1969 movie Alice's Restaurant. Arlo Guthrie's character acts like he's crazy in order to get out of military service. But despite his best efforts he seems to be sane enough for them.
He gets an homage in MechWarrior 2: Mercenaries in the form of Jake "Killer" Ives, a cheerful psychopath who was also a cheap, effective pilot, noted in his hiring profile to be a sadist bar none, with Humongous Mecha as his tool of choice for hurting other people.
Omega is a mechanical embodiment of this trope. His sole purpose is simply to kill. His bloodthirsty nature rivals even a Khorne Berserker. Who may actually be Zero's Superpowered Evil Side and/or Evil Counterpart inhabiting a separate body. Worse than that, he's exactly what Dr. Wily had intended the real Zero to be. In fact, Zero is indeed Ax Crazy, before his HeelFace Turn.
Omega's own creator, Dr. Weil, also qualifies. Destroying a whole neighborhood just to capture the Dark Elf? Check. Making humans' lives completely miserable, with the choice of either dying or living under his rule? Check. Setting a Kill Sat to destroy a human encampment? Check. Plus, going completely nuts after the Kill Sat is set and fired against himself, initiating a Colony Drop with him inside the satellite, believing himself to survive the crash... And before all that, the fact that Weil's schemes in the Elf Wars caused The End of the World as We Know It and claimed the lives of 60% of all humans, and 90% of all Reploids. He's such an incredibly dangerous person to have around that even after his bodily death, his sheer loathing and insanity leached into the surrounding wreckage, giving inanimate material the ability to feel hatred and desire complete obliteration of all sentient life on the planet. Even so, killed and diffused, he's still so thoroughly evil that mere proximity to fragments of this wreckage can drive a person to insanity, which eventually becomes a major plot point in the sequel series Mega Man ZX.
Vile from Mega Man X. Similar to Galvatron, an accident regarding his brain led to him developing a disturbing relish in not only terminating Mavericks but also causing as much collateral damage as he can while destroying the maverick in question. He also has a sociopathic hatred of X, and he revolts against authority namely to cause destruction.
Colonel Volgin of Metal Gear Solid 3: Snake Eater. Highlights of Volgin's psychotic dickery include nuking his own country, trying to destabilise the Cold War, and randomly torturing and killing his own minions simply because he can. When Snake's eye gets shot out, he practically orgasms at the sight of it.
Sundowner, Mistral and Raiden himself in Metal Gear Rising: Revengeance. Sundowner is a war-loving murderous sociopath who uses scissor swords, Mistral butchered the murderers of her parents and arranged for Khamsin's death simply because she found him annoying, and as for Raiden, it's revealed that he always was somewhat Ax-Crazy ever since he was a child soldier, but only after a Breaking Speech by Monsoon does his darker side resurface.
The Mogekos are desperate to have sex with Yonaka, against her consent, and are willing to go to extreme lengths to do so, with one Mogeko willing to blow up the entire castle and find her corpse and bang that.
Moge-ko is this Up to Eleven, who not only lusts after high school girls like the rest of the Mogekos, but torture as well, with all the Mogekos on her floor ending up as wrecks terrified in the knowledge that if they misbehave, they will get killed, and could even get killed for no reason if Moge-ko felt like it.
Mortal Kombat has a few characters known for being lunatics. Not counting brainwashed victims and near-mindless beasts, the ones that stand out most are Baraka, Mileena, and Shao Kahn.
Probably Skarlet too. Never mind the fact that what she does to her opponent is violent even by the standards of this game (sort of justified, as her powers are supposedly derived from absorbing the blood of others) she seems so incredibly unstable that Shao Kahn had to keep her in chains when he didn't need her to fight or kill someone. (And despite that, she still showed Undying Loyalty to him; of course, she was one of his evil creations from the Flesh Pits, like Mileena, who also was incredibly loyal.)
The sequel invokes this again by making it look as if Clover killed Luna in a rage. Subverted, though, as it was an accident.
Many of the bosses in No More Heroes, and more so in its sequel. Special mention goes to Bad Girl, who bats blindfolded gimps at you and beats you with a burning baseball bat, the insane superhero Destroyman, the pig-tail wearingYandere Kimmy Howell, the damned soul of Matt Helms, and Jasper Batt Jr. after taking some Psycho Serum. In fact, most of the remaining assassins come off as Retired Monsters who got Bored with Insanity.
OFF has both Japhet, who's been driven from perhaps the nicest guardian to a lunatic whose policy became "worship me or die" after a couple years of thanklessness, and any Elsen that's been deprived of their sugar for enough time, who will assault even the spectres if they dare be in the same room.
Hard to say who's the biggest example of this trope in Outlast... No, wait, no it's not. It's Eddie Gluskin, by several lengths. Even before he was committed to Mount Massive Asylum, Gluskin was a misogynistic Serial Killer who was obsessed with finding the perfect wife and mutilated several women in his attempts to create one. Being Mind Raped in one of the Asylum's experiments made him even worse. He turns his, umm, antics on the male inmates of the prison (and eventually the protagonist) and wants them to bear his children, and even has a solution to the biological incompatibilities involved.
Phoenix Wright: Ace Attorney Spirit of Justice: Typically in Ace Attorney, the murderers in each case are not examples of this trope as they usually only murder once or twice at mostnote Even Dahlia, who had planned to kill up to five people, was not blatantly out for blood. It was their way of covering for other crimes and her final attempt was to spite Mia. and tend to have motives for their crimes (even if they are petty or selfish). Even when Professional Killers with high body counts are involved, they remain professional and only kill when necessary or when ordered to do so. This game's Big Bad, Queen Ga'ran Sigatar Khura'in, is the most openly bloodthirsty culprit in the franchise. In addition to being an insane megalomaniac, she seems to revel in having executions, specifically beheadings. She also shows no qualms about calling for the genocide of defense attorney within general the country of Khura'in. While this person is able to hide this under their polite and serene regal exterior, the gloves are off during the final case with the extent of their madness put on full display. Not only does she act more hostile as a prosecutor and openly threaten to have Phoenix and Apollo beheaded on the defense bench, she even rewrites rules mid-trial to make it easier for them to slip up and do something that will allow her to call for their executions quicker once they point the finger at her.
Ignus, a fire mage, from Planescape: Torment, who nearly burned down the city of Sigil before being stopped, can join your party. Ravel Puzzlewell, who almost destroyed the city of Sigil before the Lady intervened, is connected with your character. The Xaosects/Chaosmen, represented mainly by the insane Starved Dogs Barking thugs, prowl The Hive (though you can speak to one, Barking-Wilder, and turn him briefly sane). Acaste, the Ghoul Queen, hungers for flesh, though that's pretty standard procedure for a ghoul. One of your own past incarnations was quite ax crazy and damn near impossible to kill, to boot. You can use a fossilized arm of his as a weapon.
GLaDOS from Portal is less ax crazy than she is coldly and calculatedly crazy, but it ends up having the same net effect on the people she murders. Supplemental material for Portal 2 indicates that, at first, the researchers were only able to turn her on for a few fractions of a picosecond before she tried to kill everyone. So, calculated murder is actually an improvement.
The Big BadElizabeth Greene from [PROTOTYPE] is so Ax-Crazy that she forces Alex Mercer (who is quite violent in his own right) into an alliance with Blackwatch in order to bring her down. The fact that her mind was fried by Carnival II some time ago might have something to do with this.
Aran Ryan from the Wii version of Punch-Out!! Most of the other characters emit grunts of pain or anger when you've landed a massive combo on them. Him? Deranged laughter.
In between rounds, he shouts to Mac, "Keep hitting me! I love it!!" Then, when he gets back in the ring, he punches himself several times, then makes an insane smile at the camera. His fighting style involves him running around the ring and attacking from the side, as well as jumping off the wires and trying to punch. This guy was perfectly normal in his first appearance.
"Your wimpy little punches won't even faze me." There's taking a hit and then there's getting your jollies from it.
Bald Bull from the same game gives a similar impression, both in that before matches he'll beat himself over the head for no apparent reason, and the way in which he rises from a knockdown in Title Defense mode is downright terrifying.
Brian Irons, introduced in Resident Evil 2 as the Chief of the Raccoon City Police Department, is a murderously psychotic individual. He hunts down and kills his fellow RPD officers following the T-virus outbreak, has a history of both raping and murdering young women dating back to his college days, accepts bribes from Umbrella Corporation in exchange for turning a blind eye to their unethical experiments, and is last seen preparing to taxidermy a human corpse before his gruesome death at the hands of G. The 2019 remake adds another layer to his depravity - he is revealed to have murdered an entire orphanage of children on Umbrella's orders. Granted, they were thought to be infected by the T-virus, but this was because they were used by the company as human guinea pigs, with the full knowledge and financial support of Chief Irons.
Alfred and Alexia Ashford from Resident Evil Code: Veronica. Exploring Rockfort Island and reading the files that have been left behind by former inmates and employees paints an unflattering picture of Alfred as a deeply unstable, sadistic lunatic who dabbles extensively in torture and human experimentation. Then you meet him in person and it only gets worse from there. It turns out, he also has a split personality that takes the form of his sister Alexia, who is cryogenically frozen in Umbrella's Antarctic facility. He loves her and has devoted his entire life to the service of her, in a way that carries some decidedly... ''dark'' undertones. Alexia turns out to be even more of a crazed megalomaniac than her twin brother, and every bit as hammy.
Apart from Zoe, the Baker family from Resident Evil 7: Biohazard are introduced as the very embodiment of this trope. They pursue Ethan around their property in a murderous rage, attacking him on sight for no discernible reason, and have been in the habit of abducting and killing people for years. It turns out that they weren't always like this; their insanity is due to the E-001 bioweapon (more commonly known as "Eveline", who was given the form of a little girl) taking control over their minds. (Eveline herself technically also qualifies, but as she is, in fact, a bioweapon, this is probably by design.) There is one exception - their son Lucas, an accomplished inventor since childhood, was a deranged psychopath long before Eveline arrived. As a young boy, he locked a bully in the attic and allowed him to starve to death. His shtick is constructing elaborate Saw-like traps for Baker family captives to endure. It's later revealed that he was cured of Eveline's mind control by her creators, The Connections, and has been spying on his family for them ever since. They even appointed him as Lead Researcher of their Dulvey lab in 2017, a position that he would severely abuse by subjecting its test subjects (and perhaps even its scientists) to the same treatment as the aforementioned Baker family captives.
Saints Row: Johnny Gat in 2, the Boss him/herself in 3.
Senran Kagura gives us Yomi, Ryōbi and Ryōna. Yomi's tendency to pretty much go ballistic on anyone who happens to be a wealthy individual, and her obsession with bean sprouts ascends levels of violent and unnerving. Ryōna on the other hand is much worse. She's a Combat Sadomasochist with even more emphasis on masochist. She'll likely ask random people to perform harmful acts on her just for her satisfaction. What happens if you don't? Let's just say that it won't end well for you. Finally we have Ryōbi who is an absolute sadist that gets a lot of joy out of beating down her opponents.
Ikaruga: Wait! You said you had to kill me to complete your mission! What is the meaning of this? Yomi: Those who sneer at bean sprouts will someday die by bean sprouts. That is the meaning of this.
Shadowverse gave this to Luna. After reuniting with illusions of her parents, Luna repeatedly encounters and kills an illusion of the robber who took her parents' lives, and slowly begins deriving joy from it.
Street Fighter IV's Juri is probably the nuttiest character in the bowl that is Street Fighter. Psychotic with a heavy dose of Sex Is Violence, Juri is quite the unchained animal when left to her own devices.
There's also Vega, initially from Street Fighter II, who's a little more hinged than Juri in his day-to-day conduct but the fact he's a Narcissist to the point of murdering people for either being ugly or touching/insulting his face probably says a lot about his frame of mind.
The crown king of all Ax-Crazy RPG villains is Luca Blight from Suikoden II, hands down. Nothing is safe from his wrath...not even small children. In fact, Luca rewrites the book on badass, as it eventually takes as many as eighteen of your party members, several volleys of arrows (each one shown capable of immediately killing one of his soldiers), and after all this, a duel to the death with the main character to bring him down in a suitably epicClimax Boss battle.
Super Paper Mario has Dimentio. It's not obvious at first, but about three-quarters of the way through the game he starts killing characters left and right, all the while cracking jokes and delivering lines like "It won't be so bad, I promise", in addition to being The Starscream. He's also referred to as being psychotic at least once in the game.
Barbatos Goetia of Tales of Destiny 2 had committed numerous atrocities to prove that he was the strongest warrior of all, which eventually caused him to be written out of history. Even after his resurrection, he became so obsessed with trying to prove himself that he repeatedly attempted to killactualheroes in Stahn and Rutee just to get at their son Kyle.
The main reason Jade Curtiss from Tales of the Abyss banned replicating living creatures was that when he attempted to revive his mentor who he accidentally killed through it, the resulting clone was an ultra-powerful Bonus Boss with a sociopathic personality. No wonder he was a bit apprehensive about using it again.
Zagi from Tales of Vesperia, who, at about 3/4 through the game seems to have no reason for wanting to kill Yuri other than that he wants to kill Yuri. His Ax-Crazy status is lampshaded by his one eye that's smaller than the other, possibly because he's so insane that he can't be bothered to hold them both open. In one of the boss battles against him, he casts Poison on HIMSELF! Most battles consist of him spouting random crap about wanting to kill Yuri.
Judith also qualifies, especially if you're listening very carefully to her in-battle dialogue. She giggles when ripping apart enemies and will send enemies off with an almost evil-sounding, whispered "goodbye". She's shown to be disappointed by both the 100 and 200 Man Melees (even fellow Blood Knight Yuri was satisfied by the 200 Man Melee) and frequently voices boredom if she's either not allowed to fight a lot or if battles are too short/too easy. Even Yuri's scared of her, because he fully believes she'll allow a monster to eat her just to rip it open from the inside out. She's not quite on a level with Zagi (that is, she's not Insane Equals Violent), but she definitely plays this trope. People will note that Judy's not all there in the head, including your party members. She tells the others that bloodshed excites her in a skit and she loves that blood is spilled when she inflicts pain on others. She doesn't, however, think of herself as this trope (as she's pretty hurt then annoyed when asked twice by two different people if the 200 Man Melee wasn't violent enough for her) and she has problems fighting members of your party when forced to.
Agria from Tales of Xillia. Her looks, posture, and frequent laughter make it readily apparent that she's got more than a few screws loose.
The Scout considers hurting people to be his purpose in life.
The Soldier, after being refused entry by the army during early WWII, took a ship to Europe and fought "the Nazis" until 1947.
The Heavy really likes fighting, and sometimes takes advice from inanimate objects ("What's that, sandvich? Kill them all? Good idea!").
The Pyro is a literally Ax-Crazy Psychopathic Manchild who sees the world as a Sugar Bowl full of giant lollipops and infantile cherubs. When s/he's killing people with a flamethrower, from his/her point of view s/he's just blowing bubbles and rainbows at them.
And there's the ax-wielding Horseless Headless Horsemann, the NPC from the Scream Fortress update, who attempts to hunt down and behead every player he encounters.
And Yuuka Kazami was this in the PC-98 games. Genocide was just a game to her... It's been dialed back quite a bit for the 9th game in the series... Though she is still able to worry some characters, including Cirno of all of them. And there's very little that worries that ice fairy even slightly.
And now from Word of God, Kisume has been confirmed as this much to the fans' surprise. This also makes her somewhat unusual for this trope as she is also a Shrinking Violet.
Needles Kane the driver of Sweet Tooth in the Twisted Metal series, especially his Twisted Metal Black incarnation.
The cast of Twisted Metal: Black is made up of delusional priests, hillbilly psychos, and doll-faced murderers. The sanest characters in the whole thing are Calypso, Agent Stone, and John Doe. The last two are genuinely good and sane people, who are in it for just reasons, and were institutionalized for severe PTSD and amnesia-related conditions. Still, the fact that the Big Bad is saner than most of the cast is somewhat... disturbing.
There's two more examples. One, the sadistic Flowey the Flower, and two; YOU. Yes, YOU. The person staring at the screen pressing the buttons. Not Frisk, not the Fallen Child, nobody else. Just YOU. It is VERY possible for the entire Underground to fear you as a psychotic murderer who only wants to kill every living being. And the game is a master at making you feel very, very bad about it.
The Descendants in Uncharted: Drake's Fortune are basically violent, feral zombies who kill and consume anyone who sets foot on their island. We have the El Dorado virus to thank for that.
Subverted by Eddy Raja in the same game. He fires a shotgun through Nate's car window, accusing Nate of setting him up, and rants about the island being cursed, but he has genuine reason to think he's being cheated out of his share of the treasure. And he's correct about the island being cursed- the above-mentioned Descendants are the ones cursing it.
Rafe Adler, the Big Bad of Uncharted 4: A Thief's End, is implied to not be all that sane, as shown at the start of the game when he stabs a corrupt prison warden to death, and later on, when he warns Sully that if he tries bidding on the local MacGuffin at a criminal auction, he'll leave the auction house "in a goddamn body bag". As his rivalry with the Drake brothers continues, he becomes increasingly more deranged, until he finally snaps and decides to force Nate into a Duel to the Deathwhen they're both locked in the hold of a pirate ship which is disturbingly close to exploding and consigning them both to a fiery death.
The Burning Blade clan in Warcraft II and World of Warcraft deserve a mention, as a clan of Ax Crazies that were held in check by a large group of ogre enforcers. The Horde "was not willing to employ them except in the most extreme cases, for fear that they might turn on other clans or even each other".
Though decidedly one of the less maniacal cases of insanity, it is heavily implied that the Ghosts in the StarCraft universe would've liquidated the universe if it weren't for the inhabiting implants in their brains.
Whenever I see a world untouched by war, a world of innocence, a world of lush forests and clear rivers... I really just wanna nuke the crap out of it!
Frau Engel as well. It's apparent in the first game that in forcing B.J at gunpoint to play a "race purity" game and then passing it all off as a little joke, she's not really well upstairs. Come Wolfenstein II: The New Colossus and she's even more unhinged as she chops Caroline's head off and desecrates her dead body with childish glee.
Adolf Hitler himself has become a 71-year-old Dark Lord on Life Support by the events of The New Colossus, and his senility has taken a serious toll on his already addled mind. He's become a paranoid lunatic seemingly barely aware of reality, babbling about his mother one minute and remorselessly murdering the actors for his latest propaganda film the next.
Both the game and manga of World Destruction has the heroine, Morte, who is more than happy to blow up a city. The anime version, however, gives us the resident chick Rhi'a, who has no qualms to pull out her guns when she is pissed.
Xenogears has Id, also known as the Demonof Elru. Whenever he pops up, he goes around killing everything in sight, committing genocide, crashing a "space station" (killing all the people living inside), or just behaving like a "normal" serial killer, and murdering people one by one. In reality, Id is actually Fei's Split Personality. The guy has a lot of issues.
Albedo from the Xenosaga series. The particularly disturbing "Ma-Peche" scene gives testament to this, especially when he tears his own head off to prove a point about how he can not die.
In the Japanese version, he cuts it off, but the edited scene in the English release actually manages to be more disturbing than the original.
Zombies Ate My Neighbors has both Chainsaw Maniacs and traditional ax-wielding Evil Dolls. The sounds of a chainsaw revving offscreen coupled with insane children's laughter serve to keep you awake at night.