I really like how her comment about how "a demon wearing these clothes must be a disgusting sight" hurt him. While it's obvious he cares about her to the point where he refuses to let anyone call her ugly, as stated later in the chapter, it could also be that his view of demons is far more understanding than other exorcists and humans as a whole. Thus, he dislikes the idea that a demon can't wear human clothes simply because they are a demon. Jessica had even mentioned previously that Evan has spared demons before depending on the circumstances, so it's likely that something happened in the past that made him realize just how "human" demons can be, and probably how horrible humans can be. I'd bet top dollar it has something to do with that scar of his. Maybe a demon he got to know protected him when he was young and they died as a result, leaving him with a scar and the realization that the demon situation isn't as black and white as everyone thinks it is. It's a known trope that doesn't seem to get used all that often, where the "monsters" aren't actually as bad as people think they are.
@Esper I really like that idea, and considering Evan's disposition towards "good" demons it makes a lot of sense. He probably doesn't like killing demons and desires better relations between demons and humans.