I agree with everyone else on this - don't worry about missing out on XP by resolving conflict through RP instead of combat. If it'll be anything like D:OS2, you'll be perfectly fine level-wise at endgame even if you were to be as peaceful as possible. We've also only had Act 1 to go by so far. There might be situations where NPCs/enemies come back in later Acts and either give you more to do or come back stronger and give more XP down the line. Killing them right off the bat might lock you out of those situations and rewards.
