Quite often, the only difference between an "evil" quest and a "good" quest is the motive for doing it. If you do it out of the goodness of your heart and ask for no reward, it's "good". If you do it only for the money, it's considered "evil". For example, if you want to play an evil character, tell Nalia you won't help her unless she pays you a LOT of money. I realize that's a lame way to play an evil party, but Nalia's money is green, if you know what I mean.
There are a few evil quests you can do. Go to work for the Cowled Wizards. Work for Bodhi instead of the Shadow Thieves. (Personally, I don't think that's any more evil than working for the Shadow Thieves, but lots of other people think it's evil.) Let Mazzy rot in her cell. Poison the Druid Grove. Bringing Kangaxx's body parts to him to release him from his imprisonment (and subsequently wearing his horribly evil ring) is also an evil thing to do (which doesn't stop anyone from doing it).
If all else fails, use your imagination to create your own dialogue as you play, complete with malevolent laughter.
