Both are true.

They are being cruel and sadistic for the fun of it, yes. This is true, and it must be accepted. But they were taught, indirectly, that this fun, that dominance and humiliation can be pleasant, by other people. This must be accepted also.

Unfortunately, the latter cannot he accepted without accepting the former. Most people who state the latter did not yet accept the former - it’s too much for them to see. So they are happy to spin their pretend wisdom to others, but the moment bullying happens to them, it all flies out the window, and they hate the bully, just like all victims do. When both truths are seen and accepted, one never hates the bully or feel compassionate for the bully. One becomes indifferent to the bully.