Utility is too subjective and too easily an abused standard. Would you for example not punish almost at all if they kill an old homeless drunk with no friends if they do it painlessly enough? There seems to be an intuition that that's not the correct response.

People who believe in property rights will also be concerned. Will this make it ok if I steal from you if I can show that the overall utility is increased based on my theft? You can make an argument that we shouldn't do that because once that sort of attempt becomes common society will break down, so doing it repeatedly will result in negative utility overall. But in any given single instance this won't be a problem.