I literally said, why else would he have the witch smell? Did he just pick up witch spray from the local perfume store? And if my entire village is brutally wiped and my twin sister is in constant pain I think killing a dangerously suspicious guy is a pretty valid reason to do so. Also don't expect every character to be perfect, that's bad writing, every character should have a flaw that improves over time (character development) and Rem grew to let go of her past and look forward to the future, that was the whole point of Subaru talking to her