I usually try not to respond to threads filled with penguin avatars but i'll bite...
I don't know, I do a little bit of both, in Fallout 3 I had a hard time trying to be evil, I ended up being neutral. I killed those 3 zombies that wanted into tenpenny tower, and stole everything they owned, and at the end of the game I did what
President Eden wanted me to I didn't blow up Raven Rock, and extincted all the impure deizens of the wastelands.
. I thought it best for the future of humanity, and never did see why the Enclave was supposed to be the bad guys. Baseball will live again!
In Mass Effect I killed that stupid bug queen even when I was playing as a Paragon soldier. Good riddence. A murderous, genocidal bug species wants a chance for round two? Not on my watch!
I don't play an evil character in most games because chopping up random weak peasants and stealing their useless stuff doesn't really appeal to me. The "good" route usually pays off better in these games. The exception is in Adventure mode, I chop up kids and pesants all day long, especially goblin kids all day long.
Its only a game, but I don't really get off on "evil" actions unless I can justify it in my mind or the "evil" means justifies the ends, or it just seems exceptionally fun or beneficial to do so, which is rarely the case in most morality-based games I've played.