Crovax and Ertai turned in two completely different ways. Crovax did so through a vampiric curse triggered through his own slaying of his Guardian Angel, Selenia, and completed the turn by killing Mirri and assuming the Rathi evincar-ship in Volrath's absence. Ertai, meanwhile, was systematically broken during the Nemesis storyline, becoming fully under Crovax's sway on the realization that his crewmates had abandoned him to his fate.
Urza dedicated himself to fighting against the forces of Phyrexia and spent millenia taking steps to defeat them. Once he actually found himself in Phyrexia however, the Mad Scientist in him couldn't help but admire the sheer genius of its design. He eventually turned on his fellow planeswalkers and pledged himself to Yawgmoth because he couldn't bear to destroy a plane that embodied everything he had sought.
Betrayal at House on the Hill has this as its main mechanic: the players start out exploring the haunted mansion together, but once the Haunt is triggered, one of them turns Traitor and tries to complete their evil mission before the others can stop them or escape.
A major part of the backstory of Warhammer 40,000 is Horus's fall to Chaos, which tears the Imperium of Mankind apart. A rather more minor part of the backstory is the Face-Heel Turn of the Eldar Phoenix Lord Arhra. With The Corruption a constant threat in the game universe, Face Heel Turns are a constant threat to all the greyer factions.
At least two entire factions in the game are basically made of this trope. Chaos is made up mostly of deserters from the main heroic faction, the Imperium of Man (almost everyone in the forces of Chaos is either an ex-Imperial or a daemon); that treachery began when The Paragon Horus betrayed the God-Emperor of Mankind.
The Dark Eldar are the result of a whole civilisation, the ancient Eldar empire, turning into evil hedonists full of crime-loving "pleasure cults", and accidentally creating a dark god; the Craftworld Eldar turned their back on it became good guys (well, comparatively good, anyway) but the Dark Eldar went on with the whole pleasure-cult stuff and they're now the most evil faction in the setting.
The Dark Eldar Incubi (who are essentially a dark reflection of Eldar aspect warriors who follow Arhra's teachings) have a slightly different take on Arhra's Face-Heel Turn: Both the version of the story agree that he was corrupted by the forces of Chaos, but in their version he soul remained uncorrupted, atleast long enough to order his disciples to fight him and slay him. The Incubi are still evil tho, but but unlike other Dark Eldar they atleast follow a strict if merciless code of honor.
And for that matter, Blackguards, the prestige class the Antipaladin is based upon from Dungeons & Dragons 3E and 3.5. In particular, the worst kind of Blackguard, and by far the most powerful and feared, is one who was once a paladin. A fallen paladin who becomes a Blackguard gains extra abilities, and the more levels of paladin they had, the more powerful they become as Blackguards, and if a particularly high-level paladin goes bad this way, he or she can trade in ten levels of their former paladin class to become a fully-powered Blackguard with all abilities, as well as extra abilities based on how many levels of paladin they have left after the Face-Heel Turn.
Asmodeus in the various Dungeons & Dragons settings is generally recognized as having started as a force for good before becoming He Who Fights Monsters, appropriate considering he's the single straightest analogue for Satan in D&D. On the other hand, having a Multiple-Choice Past and all, a few origin stories state that not only was he always evil, he is in fact one of the original, primal forces of evil from the beginning of the universe.
Also in Dungeons & Dragons there is a reason why the wizard Rary is known in Greyhawk as "Rary the Traitor"