I can't really ever bring myself to be evil in video games. I even go crazy defending my squadmates in Halo, I just lack the evilness required to do renegade things in games. To the point where I pass on Tali, Jack and Miranda in Mass Effect 2 because I don't want to betray Ashley even though I know that it's just a game and it shouldn't upset me so much.
I just always feel guilt, even when it is completely not needed.
That said, I certainly did have a loss of moral conscience in the original Fable, back in my old rebellious days, and I know the exact point when I lost my moral compass and went warpath.
It was the second I realized how annoying the villagers were in Fable. For some reason they drove me up the wall and I began my daily slaughter fest, killing them all mercilessly.
That said, I have yet to play Fable 2 in the evil way, so that should probably be my next goal just to see if any evil remains.