Something bad being done to someone doesn't justify those people doing the same to others.

I've had a conversation with a family member that was justifying a specific abuse by saying those people were abused first - as if that makes it okay. For example, let's say the people A, decades ago, mistreated the people B, and years later the descendants of B mistreated the descendants of A. And this person was saying that that was okay, that A should all be killed, that they deserved it.... I actually felt sick listening to what was being said. I mean, forget about doing the right thing, how about just having a little bit of humanity?! Me being abused does not make it okay for me to abuse others in the same way or worse. How does that even makes sense?! smh