Well nearly all the good ones have already been said. I'll just add my thoughts on them
- Kratos definitely, he's a horrible person, and GoWIII is possibly my favorite game ever. I still think GoW III was possibly trying to do something subversive with him, but fell flat on its ass.
- Tidus is both a horrible person and a horrible character (seriously, fuck him). Great game though.
- Alex from Prototype is a completely remorseless killing machine, though this is justified somewhat by the circumistances and a late-game twist that I really did not expect but really appreciated. The game was really fun too, haven't played it in ages though since I got my laptop.
- I'm bending the rules a bit here, but since someone already mentioned the Dragonborn, I'm allowed this pick: The Chosen Undead from Dark Souls 1. You have a bit of choice in your decisions and how it affects the world, so I wouldn't call her/him a complete blank slate. Just some things: You can kill Gwyndolin, depriving the world of its last bastion of light and hope. You're forced to kill Quelaag, leaving her sister, the ill Daughter of Chaos, forever alone and suffering in agony. If you play the Artorias DLC before challenging Sif, he will try to stop you benevolently, but you have to kill him regardless. At the end you can become the Dark Lord, dooming the world to the fate that befell Oolacile: a world devoid of civilization or sanity, where there are only ruins and the savage remains of order prowling them.
MeatMachine said:
Marche Radiuju from Final Fantasy Tactics Advanced. Quite possibly the biggest douchebag in existence, in spite of being in a pretty fun game. Him and everyone he knows gets sucked into an alternate dimension where they all have the one thing that they want the most - and he sets out to undue all of it.
Wimpy self now full of courage? Destroy the new world.
Paraplegic little brother can now walk? Destroy the new world.
Terrorized best friend now a benevolent monarch? Destroy the new world.
Other best friend now socially loved? Destroy the new world.
Best friend's alcoholic loser dad now a respected authority figure? Fuck it, destroy the new world.
Now that's an interesting pick. I wouldn't have thought of him, but his actions truly make him deplorable. I've thought for several years of him as the actual villain of the story, which certainly twists the whole scenario around. He's not trying to hurt anyone (or at least that's what he tells himself), he's nice and cooperative, and doesn't want to live in an illusion. Is he really that bad for just wanting to face reality and not escape into fantasy worlds? But when he tears everyone else down because of his selfish conviction in his beliefs, that's when he becomes horrible.