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.
That leads into the problem of everyone having omniscience, though. Every NPC ever knows where you've been and what you've done. If you kidnap an orphan nobody knows or cares about and sacrifice him/her to a demon, away from prying eyes, then people shouldn't know you've done it. The demon can blackmail you after that to expose your secret. That would be way more logical and interesting than simply giving everyone omniscience :p Or your companions can expose you if they don't like you enough and they know you've done it, etc.
