Committing genocide isn't negated because you went and helped a grandma across the street. You should be the most reviled person in existence if you commit genocide, regardless of your previous acts.
It doesn't have to be a simple scale. It could be something like traits, hidden or not, that player gets after committing some particular (good / bad) action. Similar to
Childkiller title in Fallouts. NPCs could then react to a particular title if present - regardless of other acts that player has done afterwards.
