I used to play morality based games as a goody-goody, giving money to the poor, saving stranded kitties and giggling as people poked my rotund belly. Not really sure why, but I do have a problem with being evil, even if it is just a game. That said, I have begun to play games a little more to my actual beliefs, which usually results in me being closer to an anti-hero, so, for example, if an enemy surrenders to me, I might still kill him anyway, either because he deserves it, or letting him go is too big of a risk.
Even my 'evil' characters end up having noble(ish) intentions. For example, I played through the Thieves Guild in Oblivion with a Robin Hood style philosophy, and even though my Renegade Shepard in Mass Effect is aggressive and heavy handed (he's just as likely to pull a gun on you as he is to say please), everything he does is for the greater good, or to uphold his own form of justice. This does occasionally lead to some collateral damage, though. For example, when confronting the Batarian terrorist who knocked asteroid X57 our of orbit, I refused to let him leave, even though it meant making a few sacrifices (though to make up for it, I did torture him and leave him to die).
Even if I have to do something evil to unlock something, I tend to do it in as neutral a way as I can. For example, to unlock Skorm's Bow in Fable, I sacrifice a Bandit or one of the evil henchmen, and in order to get Wellow's Pickhammer from the Demon Door which demands to see something evil, instead of killing someone, as the door suggests, I instead downed a handful of crunchy chicks, promptly followed by enough beer to make me vomit the whole lot back up, and a hefty meal of tofu and carrots. I guess being evil just leaves a bad taste in my mouth.