Bro killed the rapist that attacked his own sister who then killed herself as a result. None of it was his fault. Why in the FUCK am I supposed to empathize with the idea that he's this guilty dude trying to "atone" and that I'm supposed to care if the guys daughter wants "revenge"? Why would she want revenge? On what grounds? Like, dude, her dad was the lowest of scum and deserved what he got. She should be thanking the MC since she probably would have been next. It'd be more believable if she was the one that felt guilty because of her fathers actions that destroyed the family she currently lives with. None of this makes sense. The drama here feels incredibly forced and exaggerated, lol.
In a way I don't disagree with you, and I think there's a fair chance the girl really does feel guilty too and isn't actually planning on killing him, but you make things way too easy.
He was still her father. For all we know, he could have been the most loving and sweetest father to her, and if he was, no matter how 'right' killing him was, it would not be weird at all for her to resent his murderer.
What's more he was killed right in front of her eyes. That would be traumatic even if the person was someone you hated, all the more so for a little kid.
Yes, the fault mainly lies with the teacher, and that's something you can get a glimpse of when MC is browsing 2chan too. But, however much we like to joke about it, we do live in a society, and society has rules.
In society, killing another person is always wrong (the only possible exception would be if it was to protect yourself or someone else, and even then there's endless ways of going too far).
Of course, this 'wrong' just refers to the law. Ethically, many would argue in favour of killing a rapist, and morally most people would agree with it, and might even be willing to do it themselves. But even then, there would still be room for argument.
It's just a mess, as life often is.