I also don't see any real ntr here, but it might look that way in that guys eyes, that ended up in that tear-gas pit (hero-dude). And it is actually Kaori's fault. Let's recap:
She is childhoodfriends with that hero-dude. Hero-dude might have been in love with her for some time, but let's assume she didn't notice that. (lol) Later she fell in love with MC, when he was a nice guy, but she didn't do anything. This here is, where it is her and the circumstances fault. Surely she thought she would have time and she is a coward. And later they wound up in a situation that isn't perfect to get romantically involved. True, but if she would have confessed earlier, the drama would have unfolded at school. In the end even hero-dude would have had to accept, that she wasn't stolen from him at all. Now MC went edgelord, returned from the dead and emits this badboy atmosphere. And suddenly all faithful lovely-chan jumps his train, confesses and all that shit. That gives off a lot of ntr-vibes, even the more as dude convinced himself, that she somewhere romantically loves him, a bit? Maybe? A little? Nope, but he rolled with it. And look at MC all dressed up like a punk, murdered someone, has a slave harem. He can't be a good guy, right? Hero-dude on the other side tried to be nice and gets punished. (Later he dropped that act out of frustration.)
I don't really like hero-dude too much, but as a childhoodfriend she should feel obligated to at least talk to him. Like a sister. She must have realized his feelings by now. "I have been in love with him since some time. I thought he died, but I have a second chance and I'm gonna take it." See? That isn't too hard. Does he have to accept that? No, cause his feeling also have the right to "not lose to anyone other's feelings"! What a BS.