Well consider this: if you plan to kill everybody, you need the levels more to be more combat effective. If you're doing the "good" things and solving things without combat, you don't need to be as combat effective. Also, by focusing on the less combat aspects you get to experience things someone killing everyone doesn't.
Pacifists aren't normally as dangerous or strong as evil hobo barbarian killing everyone for power and giggles.
Instead of making do similar for each playstyle. The divergence in quests and how the story plays out should be its own reward for the "good" one. Maybe you'd get a nice family and farm and peaceful days ending vs the evil overlord with his mistress against the united world evil ending. The important thing is that a "good" play through doesn't make you feel like you lack the tools needed to reach the ending you want because of xp
