In Sidekicks, Olivia, who was a top student in Justice College but ended up in the Under Corps anyway, turned to the side of evil when Metheos gave her Phantom's Mind Control superpower. She then took on the name "Iblis".
Gale does so too after being dismissed as a sidekick.
In The Adventures of Dr. McNinja, Dr. McNinja is having trouble battling the evil Frans Rayner, because all the McNinja clones (long story) are triggering Conservation of Ninjutsu. So the Genre Savvy doctor does the only thing he can — team up with Frans and start taking the clones out himself. Yes, Face–Heel Turn as a fighting tactic. The clones were going to die either way, and Doc would have died with them if he hadn't changed sides. Besides, he went back to Face when there were no more clones, because Conservation of Ninjutsu was no longer in effect (and Rainer would have killed him had he stood quiet).
Eridan from does this after deciding that the fight against Jack Noir is completely hopeless and as the "Prince of Hope" he believes he is the most qualified to determine if all hope is lost. He figures the only chance for survival is to pledge his loyalty to Jack. He then proceeds to K.O. Sollux, kill Feferi, and finally kill Kanaya (while destroying the key to restoring the Troll race) when they try to stop him before he escapes.
And then, not long after, Gamzee runs out of sopor slime pies and goes insane (or sane, considering that sopor slime pies are a troll drug), which causes him to remember his destiny as a murderous Subjugglator, and is later seen working for Caliborn/Lord English.
Synn from the The Law of Purple performed one of these before the start of the comic. It didn't take very well, tho, since it took little more than a few words from his old best friend before Heel Face Turning back.
Ruby of Sticky Dilly Buns is moralistic to the point of sanctimoniousness when she first appears, and as a result acts as The Conscience to title character Dillon once or twice. Unfortunately, she then develops an obsession with yaoi, and starts trying to make her Guy on Guy fantasies happen in real life — usually by exploiting Dillon's trust. However, her own conscience doesn't entirely disappear.
Wrestler Roxy of Ménage à 3 executes a classic in-ring Turn, attacking her own tag team partner from behind.
Corrick does this unwillingly in Plume, as he must protect whoever's wearing his amulet and Dom manages to get ahold of it, forcing Cor to turn against Vesper.
Hotaru ends up on the side of evil when she's possessed by Chaos's final avatar, Servant Chaos.
You know how in the canon Sailor Moon Chibi Chibi really pushed Usagi into destroying the Galaxy Cauldron in order to stop the cycle of good of evil? Chibi Chibi is none other than a reincarnated Usagi, who decides that she can't bear to be in a world where evil can't be completely eradicated.