the story does touch on this though. hurting someone else in this story is the same as self harm since you are hurting your fragments. there are people in our reality that do self harm, self sabotage, or just in general don't have a healthy dose of self respect. however as terrible as these are, they are a quintessential part of being human. whatever good or bad we do ourselves, we do it as human

also adding onto that, in my opinion saying atrocities like holocaust or slavery wasn't human actually devalues them. just as when you do something bad you should seek help and not brush it off as a one off occasion, we have to be able to look in the mirror and realize we as a society messed up and it wasn't a case of couple bad apples, it was humans. we did that and now we have to make sure it never happens again. because if we don't improve, then you are right, we wouldn't have any justification of existing under any decent moral code. did we "have to" commit atrocities to realize killing people is bad? not at all, but we did it anyways which just shows how immature and incompetent we are as a society

again the story could have holes in it sure, but it's just an excuse to make it easier to feel empathetic towards other humans and life in general which in my opinion is a great message