Notice also that an evil guy knows that he can first do things in a good way to get the rewards, then let his killer instinct come out...
When i played an evil character, i gave him something in his personality that helped me feel better and keep his reputation decent: he doesnt kill kids, and he helps them. That made him more human, and i didnt want to kill kids (even i a game)... so i did a few quests (kid-related quests like the cat in the waterfall) in a completely good way
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"Iīve been captured... and I donīt care" -Ertai, Captive of the Blinding Angel