Not quite! Arthur's definitely hurt other people before, but he's always thought to himself, 'I have to save others without hurting anyone.'

Now, his way of thinking would switch to, 'I'll only be able to help others if I hurt whoever's endangering them' which would essentially be rewriting his goal to avoid being a monster. Basically, Emilia's death is the one thing that's teaching him he doesn't have it in him to be a good hero.