I think the game really doesn't work if you're evil. Nearly all the sub-quests involve helping someone. The only way to role-play it evil is to rationalise, "By helping this person, I'm getting loot and good will, which will make it easier for me to do evil elsewhere." Plus, being evil automatically leads to a low reputation and all the shop-keepers give you bad prices. I think you should be able to be SO evil that everyone's scared of you, and you actually get better prices.

Not that I can ever bring myself to do evil things. I just can't do it. And, being a lawful good paladin, I never let anyone evil anywhere near my party. It doesn't feel right. I'm chaotic good this time, so I'm preparing myself for a bit more moral flexibility to my actions.