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 — not to mention that 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 batshit insane after he realizes his Resureccion 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.
Mabashi, in the Bount arc, was so against sucking living souls that he actually had to be force-fed his first. Afterward he goes completely and totally Ax Crazy, burning through any sympathy we might have hadvery quickly.
Recently, in One Piece, Blackbeard stole the Gura Gura no Mi from Whitebeard's body, and goes off the rails, threatening to destroy the island of Maineford, kill everybody, etc. It takes the arrival of Shanks to sober him, but it's likely a temporary solution for now on 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...
Comicbooks
Superboy Prime, starting with Infinite Crisis and getting steadily worse thereafter.
As seen in the page image, Hal Jordan after his temporary Face Heel Turn and turning into Parallax.
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.
Film
Palpatine really tears into Mace Windu in Episode III of Star Wars and then Luke in Episode VI.
Heck, it's pretty much 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.
Skeletor pulls one of these in the Masters of the Universe movie. Oddly enough, his godlike power does not allow him to win a swordfight against He-Man.
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.
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 basically goes from being a gleefully psychotic man to being a gleefully psychotic god.
Sephiroth does not become insane as a result of his evil nature. He becomes evil when he goes crazy, and he's much less of a nihilist, and much more of an Übermensch/A God Am I type villain. Plus, his plan isn't random destruction, but instead a very carefully thought out plan in which the destruction he plans is vitally important to him achieving godhood.
At which point he would simply use the rotting husk of the dead planet as a giant space ship until he finds another inhabitable planet, where he would then use his godly power to start life over from scratch. And there you go (Somehow he seems to have gotten the idea that all Humans Are Bastards and that this justifies him destroying the world and starting over on another planet...I blame Hojo)
To be totally fair, you only find out about his planet-is-a-spaceship plan if you watch Advent Children. And even then no explanation is given as to HOW he is going to move the damn thing. As powerful as he would be, I would think that even he would have a problem pulling the planet he was standing on out of orbit. Maybe the weight of his ego would cause the earth to fall out of orbit...
The fact that it's never outright stated how is a bit of a problem. It's possible to figure out if you do a bit of research, hunt down the side-material (some of which, sadly, is Japan-only) and then connect the dots. It has to do with the Geostigma: basically, Sephiroth causes Geostigma by spreading Jenova's remains through the lifestream, then when the Geostigma kills someone it corrupts that persons soul and places it under Sephiroth's complete control. He's basically eating the souls of anyone killed by it. The plan then is to use the corrupted lifestream to take control of the planet and pull it out of orbit. The dark clouds descending from the sky during the final battle are corrupted lifestream reaching down to grasp the planet. He just needs to get a good grip, and then use the souls of everyone killed by the Geostigma as a giant lifting crane to move the planet. Then the freezing cold of dark space will kill everyone that's left and leave Sephiroth to eat the rest of the lifestream.
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.
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.
To explain the second page quote from The Dungeons And The Dragons, a group of D & D players have invited one of their mothers to sit in on a game to prove that roleplaying games aren't evil like Jack Chick says. Of course it turns out that it's she herself who turns evil. We're talking The Exorcist on steroids evil.
