I really liked Prototype, which is available on all platforms. It makes wholesale slaughter a very hands on experience and you can turn the streets of Manhattan into a body horror abattoir.
Fallout 1 and 2 and New Vegas offer up some great mechanical and narrative nastiness. Fallout 3 has options, too, but the Black Isle and Obsidian Fallout games have more teeth than Bethesda's entries.
Grand Theft Auto and Manhunt (series) by Rockstar give you some good morally blank sandboxes and psychotic, depraved stealth games, respectively.
Saint's Row 1 and 2 were pretty solid, though 3 was a bit too whacky to actually feel properly evil in.
Vampire the Masquerade: Bloodlines is a great evil character RPG, true to the White Wolf pen and paper RPG. Cool, ambitious game let down by a few technical issues. Worth playing.
Off Good Old Games.com you can pick up some older titles, too:
- Postal 1 (though isometric, it was considerably more 'evil' than Postal 2 or 3's silliness)
- Syndicate (the original Bullfrog one)
- Messiah (you are a cherub sent by a wrathful god to punish the sinners by possessing their bodies and making them kill each other)
Finally, as RTS, 4x or TBS goes, various historical games like the Total War franchises let you play pretty brutal warlord autocrats all throughout history. Relic's games are good, too: 2 Nazi armies, the Wermacht and the Panzer Elite. Their Dawn of War warhammer 40k titles feature *mostly* evil armies, too... Necrons, Tyranids, Orks, Chaos, etc.
-----
Anyway, I hope that gives you some ideas dude, I feel your desire to be a bad guy and have enjoyed games that do that for me. I think that's why I gravitated towards being the storyteller / Dungeon Master in D&D and other RPGs... got to play all the villains >
