Good point. I think that there are people who treat others poorly inadvertently; they don't mean to but someone still gets hurt.

I think there are people who hurt others for a variety of reasons. For example, I tell my high school students that high school romances usually end with someone being hurt. I don't think this means that either student is "bad", nor mentally ill.

And, there are some people who actually enjoy hurting others or don't care if others are hurt. Some people think this is a sign of mental illness. I am not convinced of this; I think some people are just selfish. But obviously opinions on that differ.

I think that some people treat others badly because they have themselves been treated badly.