So why -should- you be evil? Because you know what's best and will subjugate or kill anyone who thinks otherwise. So get on it!

Okay, but by that logic, I should just kill everyone, because none of them can be subjugated.
My point is that if Larian wants us to play evil, our character's need a valuable reason to choose that path. Homicidal Maniac and Goblin Fetishist aren't the best draw for most people.