Double-dipping will most likely make you evil anyway over time. So some stuff will get automatically locked away if you try to double-dip. And even if you could manage to stay 'neutral', really good and bad stuff would stay locked away.
Questing for a evil one makes you infamous, killing him afterwards perhaps gives you bit good back, but will it compensate the bad influence from the quest? On the other hand helping someone with something good will give you fame, but killing him afterwards will be most likely worse. Evil acts have always bigger influence than acts of good.
