Look, I'm going to assume you're not reading in bad faith, so I'm really at a loss at this point.

I don't think I've been unclear that there's a distinct difference from being "harmful"--in the sense that sin eaters and mankind are incapable of co-existence because of the basic function of sin eaters--and "evil"--which are actions designed to cause unnecessary suffering to those the actions are directed at. Titania was not locked away in the castle because their people thought they were being too mean or too violent to others, they were locked away because they turned people into sin eaters. Vauthry does not do that. I would expect a pure sin eater Vauthry to just turn the entirety of Eulmore citizens into sin eaters and continue their spread, as every other sin eater in the game does.

The things Vauthry is hated for are things he would be capable of doing without being any part sin eater, so how can you possibly blame him being a sin eater for people not feeling sorry for him?