Why would people with mental illness purposely BULLY someone else who suffers from it as well?

They must know how people (especially with PTSD) will react to feeling cornered, threatened, and belittled.

Why are people so mean? Are they angry and bitter about the way their lives turned out?

Do bullies think they are entitled to treat others poorly? Does bullying others make them feel smarter? More powerful?

Maybe it's misdirected anger.

I hate to see my friends hurting because of people that bully.

I hope my friends know it says more about what's going on in the bullies lives (or lack of) and that it's NOT a reflection on them, although the bullies will always try to deflect that responsibility back on them.