Magical Project S When Misao, normally a shy, timid girl, transforms into her evil magical self, she starts to do everything she couldn't in powerless form — and as pixy misa she starts to sing about how powerful she is (she sings in the middle of the street, as everyone assumes a WTF expression) with hilarious results.
Muska in Miyazaki's Castle in the Sky qualifies at the climactic events of the film when he takes control of Laputa's powers and both kills the general and his soldiers and destroys the army's battleship with an army of robots, gloating, "A superior being such as myself has only one option—burn them!"
Nene in Blue Dragon does this when he absorbs the main characters' powers.
Death Note: Light Yagami started out as a kid groomed for success. His dream was to join the police and assist his father, the Chief of Police. He then became the popular guy in high-school because he was smart, studious, and Bishōnen. He was even smart enough to be one of the top scoring high-school students in all of Japan! Then he found a Death Note...
In Bleach, Tosen goes absolutely insane after he realizes his Resurreccion gives him eyesight. More than he already was, anyway.
And Aizen has gone off the deep end as well, doing incredibly dickish things for no reason except that he can, and loses it not once, but twice, when Gin succeeds in killing him and when he discovers Ichigo's more powerful than him now. Yes, that's in the correct order.
Kariya, the Bount leader, attacked his allies in Soul Society and had to be restrained by two of his followers.
During the Paramount War in One Piece, Blackbeard stole the Tremor-Tremor Fruit from Whitebeard's body and goes off the rails, threatening to destroy the island of Marineford, kill everybody, etc. It takes the arrival of Shanks to sober him, but it's likely a temporary solution for now at that.
In Record of Lodoss War, the evil magician Wagnard becomes even more megalomaniacal by the final episode, cackling with mad glee as he blasts at Parn and Ashram with his magic from his scepter until Ashram eventually kills him. Even so, he gets to have one last bit of scenery to chew on as he screams, "All Lodoss is doomed! Hahahahaha! Nothing will remain! NOTHING! Hahahahahahaha!"
Sasuke Uchiha from Naruto initially wants power to surpass his treacherous brother Itachi, but with time, he spirals dangerously out of control. Too much exposure to evil and Mind Rape twisted him into an Ax Crazy lunatic- learning Itachi pretended to be a villain so his brother would kill him to atone for slaying his clan, but actually still loved him was what made him crack. Now, he's to the point where he wants his whole village wiped out. Unless... his revived brother can convince him otherwise...
Turns out it just made him more convinced to destroy the village.
Happens in Mega Man, when Mega Man starts getting the Robot Masters' powers he starts to get increasingly cocky and arrogant, to the point where he declares himself invincible. This results in Dr. Light having to contain him while he throws what amounts to a super-powered temper tantrum. When he calms down, he's so horrified at his response that he asks Dr. Light to take them away.
An act of Fridge Brilliance on the writer's part- this explains why Mega Man's acquired powers don't carry over into the next game- he's only borrowing them until Wily's beaten.
In the original Franchise/Transformers comic book series by Marvel, this happens to Starscream after he absorbs power from a mysterious satellite called the Underbase and becomes so strong that he's able to take on the combined might of both the Autobot and Decepticons combined and curbstomp them both. He's only defeated when Optimus Prime tricks him into absorbing the rest of the Underbase's power, which turns out to be far too much for any single Transformer to hold.
Palpatine really tears into Mace Windu in Episode III of Star Wars and then Luke in Episode VI.
It's part and parcel of a Sith's power in the first place- as opposed to the Jedi, who control the Force through serenity and self-discipline, a Sith unleashes it through raw, destructive emotion- and as such, even a normally self-controlled Sith like the Emperor will go straight to Large Ham mode when using his powers.
In Disney's Aladdin, Jafar really hams it up after wishing to become the world's most powerful sorcerer and, when even that isn't enough, he wishes to become a genie himself and hams it up even more until he notices the BIG downside to being a genie.
In Galaxy of Fear, the first clone of herself that Tash Arranda meets is gleefully fascinated by the dark side, willing and wanting to kill and demonstrate her power. She has a wicked gleam in her eyes.
In the Buffy the Vampire Slayer episode "Revelations," renegade Watcher Gwendolyn Post tricks the Scooby Gang into recovering the evil artifact the Glove of Myhnegon, and promptly becomes this trope.
after Miss Post has put on the Glove of Myhnegon and absorbed its power
Only after accessing the tainted power does Castiel proclaim himself a new god and threaten his friends' lives. In season seven, Dean's fears and continued willingness to kill Castiel are borne out by Castiel smiting angels and humans even before the voracious Eldritch Abominations inside him tip him into a murderous rampage and he nearly explodes before the unkillable monsters escape into the world.
Once Upon a Time: The longer Rumplestiltskin has his powers, the worse he gets, especially after he loses his Morality Pet son. (While Mr. Gold, Rumple's depowered human Storybrooke counterpart, remembers who he is, he is much calmer and in control of himself than his fairy tale self.)
In Knights of the Old Republic, most of the students at the Sith Academy have this to various degrees. In fact, one way to get inside is to murder one such student, since he's just being a total dick and killing people at random because he cant get over how easy it is. If you turn to the Dark Side, you can force one of the main characters to kill his best friend because she won't swear loyalty to you. Although he'll snap out of it and turn on you if you keep him in your party.
"I've tapped into the ultimate power. Observe...! Such magnificent power! You are like insects to me!" -Kefka, Final Fantasy VI
Kefka is a subversion, as his personality stays exactly the same after gaining ultimate power - which is bad, because he was a gleefully psychotic monster to begin with. Kefka goes from being a gleefully psychotic man to being a gleefully psychotic god.
Esher in Myst V: End of Ages does this if you make the mistake of giving him the tablet.
Exposure to Phazon causes this in the Metroid Prime series. In the third installment, Samus gets to experience it herself. It's still better than the alternative, though.
What makes Phazon even more insidious is that early stages of The Corruption can be shrugged off by expelling excess Phazon by shooting randomly, which is highly likely to cause any and all allies to think you've either gone berserk or already succumbed to this trope.
In Mega Man Zero 2, as the final boss Elpizo is transforming into his One-Winged Angel form, he lets out a piercing scream, and then yells (in Japanese) "MORE POWER!!!" in a demonically distorted voice.
In Overlord II, when you drain Queen Fay of her magic, she slowly begins to be corrupted by your dark magic, begging for you to continue and yelling at the Elves and Pixies for trying to save her. By the end she's driven completely insane and becomes a full-on Fallen Hero who joins you.
It is a quite common effect of demonic magic, especially for careless users. One boss in Magister's Terrace which, after charging up with a bit of it outright screams "YES! I AM A GOD!". His boss, Kael'thas, gets "unlimited power"-style quotes in both his appearances.
In a meta vein, whenever any class or skill ends up buffed up significantly, players of that class will go on this trope completely. In one case, Shamans getting the ability to cast Chain Lightning with no pauses made the resulting light-show look just like Palpatine's Force Lightning. Flooding of Youtube with mashups of his quote from the page top with in-game footage promptly followed.
This is one of the more... legitimate reasons Blood Magic and dealing with spirits is forbidden for mages in Dragon Age.
In Odin Sphere, the spell of Darkova transforms its user into a powerful three headed demon beast. The user is then cursed with a thirst for blood and pain, compelling them to destroy that which they love most.
Albedo from Xenosaga firing Proto Merkabah at Second Miltia is an example of this.
Yeager in Nodwick acted this way when he had won the power of "The Quirkening", going so far as to threaten to kill Nodwick... well, ok, he's normally like that, but this time he meant to use his Reality Warper powers to really screw him over.
Vaarsuvius in The Order of the Stick, after making a Deal with the Devil. At least initially. Soon after the character starts actually using that power, they feel nothing but frustration as resolutions to some plot threads have been reached without having them use it. And then finally when they try to fight the Big Bad, an Evil Sorceror that has technically less power, but is more than equal and is much more proficient in it's use where it matters, they get stomped and lose all of that power. Though while the end result was far short of what the character imagined, they still managed to achieve a couple important things - and could have done more if not for the power-drunk rampage.
Chloe in Eerie Cuties, when she found a demonic Amplifier Artifact made for mature succubi — which is what she isn't, yet. She quickly grows powerful and restless, sliding out of control and from an innocent crush to rather crazy to full demoning-out with urges to corrupt pretty quickly, ending up with breakdown and then bad hangover after this "fun" ends.
"I have all the power in the WOOOOOOOOOOOORRRRRLD!!"
He was a raging sociopath long before the comet, it's just now he had enough firepower to really let it out.
Being voiced by Mark frakking HAM-mil helps.
In the Season 2 finale of Justice League UnlimitedLex Luthor becomes this way after merging with Brainiac. He mentions that in that state, he has power, and he likes it. Throughout season 3, he constantly tries to reactivate Brainiac from a small piece of technology-covered rock to regain that feeling.
In My Little Pony: Friendship Is Magic has this happen with Trixie while wearing the Alicorn Amulet. She was a pretty Large Ham to begin with, and the Amulet not only drastically increased her power but corrupted her mind. In this case, it's reversible because all it takes is removing the Amulet and giving her a bit of time to come off the high.
Being drunk in general can cause people to make some pretty dickish moves, become drunken and violent, consider themselves invincible and take some serious risks with their own personal safety and that of others.
The Emperor Caligula was a pretty good emperor at first. Until for reasons that are still being discussed up to this day (ending uip brain-damaged after a seriousl illness, wanting to provoke the uber-powerful Senate, etc.) he started doing dumb stuff like giving his favourite horse a position on the senate just to make the other senators feel stupid, proclaiming himself a god, dressing as Jupiter, having the sea whipped for its disobedience and annexing a sacred Jewish temple. Ironically, it was only after he proposed a golden statue of himself constructed in the aforementioned Jewish temple that the Praetorian Guard whose sworn duty was to protect the emperor no matter what, decided to do what they did best in times of great need: kill the emperor.
According to this article from Cracked, hate makes you high. Literally; the chemicals released by your brain when you indulge in hate are the same ones that are released by smoking crack.