Following in Bioware's tradition of casting a prickly and arrogant but powerful female lead, Bastila is a Jedi savant who's skill backs up her cockiness. What she lacks in combat experience she makes up for by stabbing people with a double-ended laser sword. But when Darth Malak kidnaps the Jedi Princess, it's up to your Jedi-Mario to traverse the space castle to save her. As the story goes, Sith-Goombas are stomped, pits are leapt over and when you finally reach your damsel in distress, she challenges you to a one-on-one duel to the death.
Wait, what? Yeah, Bastila's gone and turned to the dark side to serve as the Sith lord's apprentice (because that always works out so well). After the player spent the last 30 hours learning how important she is to the Republicnot to mention how important she is to your charactershe hands you her two weeks' notice in the form of fiery, space-magician death.
Over time, gamers develop a resistance to videogame plot twists. If a powerful character seems suspiciously buddy-buddy, most gamers will peg them early-on as a snake in the grass. That's what makes Wheatley's arc in Portal 2 so deceptive he's not your kind old master, or your best friend, or a love interest (barring some interesting fanfictions), he's just a lovable idiot. He's the comic relief in a research lab full of pointy, smashy, corrosive bits, and is so pathetically impotent he needs a biped like you to carry him around. You never expect Wheatley to become a threat to you because he's never a threat to anyone.
That is, until a by-product of your escape attempt happens to land him in GlaDOS's big control chair, and all of a sudden the power balance is flipped: the little ball of insecurities isn't so adorable now that his inferiority complex is powering the entire facility. And no sooner does he get a taste of power than he's re-appraising your work relationship in an uncomfortably murderous fashion. For all the suffering that GlaDOS put you through, now you find yourself pitted against an entity even more dangerous: an all-powerful moron.
Kefka is arguably the greatest villain in the Final Fantasy series. Firstly, because he blows up the fucking world; secondly, because he's the series' first villain to be just a man, not a demon or an alien, just a man with a messed up brain; and most importantly, because he serves no will but his own.
For the game's first act Kefka is set up as just a middle-management flunky standing between the party and what seems to be the real villain, Emperor Gestahl. He cackles comedically and stomps up and down when the party slips out of his reach. You can almost picture him screaming about how "he'll get you next time, Gadget!" But before long it becomes more and more clear that, when the leash is loosened, Kefka is completely bonkers.
He slays the national hero and general of his own army, y'know, just 'cuz. And remember that unnecessary act of bio-terrorism? Classic Kefka! It isn't long before he's completely uncontrollable; Kefka doesn't even seem all that interested in power, just chaos for its own sake. So when his Emperor finally tries to sit Kefka in the corner for a time out, Kefka decides that it's high time he blew this popsicle stand
and by popsicle stand he means entire planet.