Then I really really recommend The Chronicles of Riddick. That game makes you feel like such a badass. As in, not psycho, or bad guy, but just badass

They let you out of Guantanamo Bay already?

I've tried to be really nasty in Mass Effect 2, but no matter what, I keep getting the "good" score going up instead of the "bad" one =(

Terranigma for SNES.

That's the most badass one.
***SPOILERS***
If you play it mean, Sonic takes out a chainsaw and eviserates his foes. All his party members turn their backs on him in disgust. Which turns out to be the wrong move for obvious reasons.
***END SPOILERS***
(The preceeding may or may not be even slightly accurate.)
As someone mentioned KOTOR, the sequel could also count. Not Bioware, but a nice enough game in its own right.
You can also be a jerk in Planescape: Torment, one of the RPG greats.

Try any of the old Infinity engine games, Baldur's gate and the sequel particulary, then you can play a mean SOB and gather a party of SOBs along the way! Also the Icewind Dale games let you create a party of evil characters right off the bat. There's branched dialog for them and everything.

+ 1 for Chronicles of Riddick
+ 1 for Bioware. KOTOR 1 in particular because of who you are playing.

That whole Paragon/Renegade thing is broken. You can do bad things by using persuade and get Paragon points but then doing good things by using intimidate gets you renegade points.

Same here. That is probably the worst part of ME 2, a big chunk of renegade points in that game are accumulated by being a complete asshole to your crewmates, something I can't quite bring myself to do.

Legacy of Kain? Overlord?

What about the Fallout games?

The Geneforge games allow you to be evil as well. The best way to achieve the "evil" requirement is to join the Shapers and do quests for them while murdering entire villages of rebels/renegade Serviles...especially once you get some of the higher level creations and have enough Essence to maintain an army (6-7) of them!!
You can be evil in the Space Rangers games as well. For example, a merchant ship calls you to help them out against a pirate you can decline and kill both the ship AND the pirate attacking it >:-D. Of course there are many, MANY other ways you can be evil in that game, it just depends on your imagination.

You can be a bit of a dick in The Witcher. Not outright evil though, but you can be quite nasty at times.
Also, Ultima 8. Bad Avatar!
My favourite is Civilization 4 at the moment though, capturing enemy cities and then razing them to the ground! MUAHAHAHAA!! :)

[url=]http://kael.civfanatics.net/[/url] - Fall from heaven 2 mod(sorry I can't help myself, I've probably mentioned this mod about a zillion times at this point.)
This mod not only allows you to play as evil civilizations it also lets you summon a demon ally that will grow stronger as war rages across the planet. Some of the souls of the dead actually get feed into the demons army. Once summoned it also gives you the choice if you want to switch over to playing the demons. Its also got a ton of other evil things like vampires that can feed on their own population to grow stronger, a cthulhu style religion etc.
Now I'm going to have to play this mod yet again damn...

Second Life :P
Alternatively, EVE Online.
EDIT: Oh yeah, and we can't forget about Black and White. Beat your pet, make him throw villagers into the ocean, and make him eat his own poo. :D

Doesn't Populous also allow you to live out your angry, jealous God fantasies or is it just me??