Hal Jordan, Green Lantern poster child, underwent a psychotic turn into a being that wanted to reshape all of existence. Later explained and Retconned, but still heavily controversial.
Two-Face, in the Batman comics is a good-guy district attorney who turns into an evil criminal.
Cassandra Cain to a lot of disapproval. After spending 76 issues delving into her extraordinary psychology and fleshing her out into a complex character with high moral standards, DC tore her down and made her into a drugstore-variety Dragon Lady who started killing people like roaches. After an IMMENSE backlash, they very "cleverly" revealed that she was actually brainwashed by Deathstroke the entire time.
In a particularly bad example of Executive Meddling, DC planned to turn Captain Atom into supervillain Monarch. When readers guessed the plan ahead of time, they changed their minds at the last minute and made the character Hawk murder his partner Dove and don Monarch's armor. Then, in a series of decisions years down the line, they effectively reversed it, giving Hawk the new handle of Extant, putting Captain Atom into Monarch's armor and creating a new Hawk and Dove team.
In Convergence #6, Superman and the rest of the heroes from Kingdom Come. While the story they come from arguably ended the Dark Age of Comics and brought back idealistic heroes, These versions agree to serve the openly Tyrannical Deimos and fight the heroes of Earth 2 if it means their own city surviving. This is turned back immediately after Parallax murders Deimos, as they had no reason to be evil anymore.
Subevrted in Death of the Family. Harley Quinn is blackmailed into helping the Joker when he threatens to destroy Deadshot's corpse (Harley doesn't know that Deadshot is not dead though), after realizing Harley has feelings for Deadshot and is quite pissed off about it. If you did not read Suicide Squad, you would not know about it and think she had gone evil again.
Mandrakk, the Dark Monitor from Final Crisis, was a Lovecraftian Horror ultra-vampire who wanted to feed on the life blood of existence itself until it had been bled dry. Saying he was the Ultimate Evil is not pushing the envelope. If what has been suggested - namely that he originally was the Monitor, the heroic mentor figure who laid down his life so the Universe might live in Crisis on Infinite Earths - is true, then this might be the ultimate FHT in comics.
Infinite Crisis did this to Alexander Luthor, Jr. and Superboy-Prime, two of the heroes from Crisis on Infinite Earths. Alex turned due to jealousy towards Prime and Kal-L, the Golden Age Superman; both were able to have a childhood and future Alex did not. Alex later convinces Prime to help him with the idea that he could revive Earth-Prime and save his family. Prime ends up fully turning after he knocks Pantha's head off.
Mary "Damn" Marvel provided an interesting contrast as a Golden Age teen amidst modern comics characters. Unfortunately DC decided to make her turn evil. Her clothes even turned black.
Name a Teen Titan. Chances are good he/she has tried to kill the other Titans at some point. Raven, Jericho, and Beast Boy (yes, even BB) are some of the more well known ones.
Raven and Jericho have frequently gone insane and/or evil to the point that it has practically become a Running Gag. Rose Wilson was drugged into becoming the Ravager by her father Deathstroke. Roy Harper became a psychotic anti-hero after he was mutilated and lost his daughter. Donna Troy was revived and brainwashed by the Titans of Myth after she was killed by the Superman robot. Cyborg nearly lost all his humanity when he became Cyberion and almost destroyed the moon. This is just the tip of the iceberg for the Titans.
Tigress in Young All-Stars after her death and resurrection at the hands of Gudra the Valkyrie, which was meant to explain her origin of becoming the Golden Age villain the Huntress.
Avengers Arena: Apex is the only one of the kidnapped teens who is actually willing to play Arcade's game, killing Juston to steal his Sentinel, seizing control of Deathlocket and the Darkhawk armor with her technopathy to carry out her plans, and using the latter to kill Nico.
From Fantastic Four, Sue Storm believe it or not. She once went mad and became a villain named Malice, who was hell-bent on killing her former teammates. When she recovered, this was one of the biggest contributions to her changing her name from the Invisible Girl to the more mature Invisible Woman and Taking a Level in Badass.
Following the Superior Spider-Man arc, Peter's former friend Liz Allan has been seen in a new alliance with Norman Osborn as he attempts to establish a new identity and corporation (now that his identity as the Green Goblin is no longer a secret). Whether she is doing this entirely out of free will is unknown, but it's very possibly that the biggest reason is to ensure a better future for her son.
Colossus, after his sister, Ilyana, died. It didn't last long.
The longtime member Bishop - after years of looking for a mysterious traitor who was supposed to kill X-Men, he betrayed them himself, for the same reason he joined them in the first place - to stop a Bad Future from happening.
In Runaways, Chase briefly went over to the dark side, attempting to form a pact with the Gibborim in an effort to resurrect Gert.
During Civil War, the New Warriors became pariahs after being blamed for a disaster that killed over a hundred people. One of their former members, Hindsight Lad, decided to turn on his former teammates, giving away their real names and addresses to the angry mobs in the hopes that he himself would be spared.
Javi in Negation goes through this after Charon brought him back from the dead and convinced him that his conquest of the Negation-verse and his pending invasion of the main CrossGen universe was a good and necessary thing. Javi, a religious man who had questioned his faith even before his abduction to the Negation-verse, came to look upon Charon as a god, one in whom he could place his whole trust in, without question. He becomes a Knight Templar as a result.
In Archie Comics' Sonic the Hedgehog, the most notable thus far was Fiona, who turned against the Freedom Fighters in #172 and joined Anti-Sonic/Scourge in Moebius (formerly Anti-Mobius).
Espio turned against the Freedom Fighters and Chaotix to join the Iron Dominion, but returned. It turned out to be a ruse, but it wasn't exactly a harmless one.
And before either of those two... Drago Wolf and Sleuth Dog.
#178-179, though it could just be a political debate in which both sides use violence instead of words.
Geoffrey St. John appeared to have one in issues 219-220, but this is actually a subversion, as it was soon after retconed that he'd actually been working for Ixis Naugus since before either of them were even introduced, making him The Mole instead.
Irredeemable is based on this concept with the Plutonian, a Superman-esque character, snapping due to the pressure and becoming the ultimate villain.
In All Fall Down, Pronto undergoes this in exchange for new legs as fast as his old ones.
In Sonic the Comic, Nack the Weasel is introduced as a member of the heroic Chaotix - but by the end of their introductory story he's sold them out to the Brotherhood of Metallix, an army of spectacularly evil robotic copies of Sonic. Admittedly a loose example, as a) he'd clearly already turned before the story started and b) the character was always a villain in the Sonic games, so seeing him as a hero first is the unexpected bit.