One of the core beliefs of the West's major religions is that evil is a corruption of good. The well-known seven deadly sins of Christianity are all distortions of love. Lust is a corruption of physical pleasure (no, not all such pleasure is sinful). Vanity is a corruption of love of beauty. Envy is a corruption of love of experience, pride a corruption of love of self, and so on.
A major trend in 20th-century literature and film has been to demonstrate this in the form of sympathetic villains. Unlike with pure villains, the original nobility of these villains is shown. It is revealed how their perceptions and desires have been corrupted, how pain has led them to reject their good inclinations, how some noble instincts remain. The audience roots against such a character, but is not wholly against that character.
So it's perhaps surprising that sympathetic villains do not appear more often in games. A notable exception is Bioware's Neverwinter Nights, with one character corrupted by misplaced faith and another turned by pain and bitter regret. But such a scenario seems rare in games.
Anyone who reads this blog regularly knows that I'm a fan of fables and mythic characters. But complex characters have their place, too. Why do you think games have gravitated so exclusively toward stick figures and moral extremes?
Mythic villains have been making a comeback in recent decades, particularly through comic books and their film adaptations. Perhaps games are just following a new trend in fiction?