Determining whether anyone’s anger is justified or not is a useless endeavor. The only measurement of anger, in my opinion, is whether it is useful. I most cases, it isn’t.
The only time anger is useful is to physically attack someone. So, if a person is under physical attack, and attacking back is the only option, then anger is useful.
Getting angry over something in the past is useless because nothing can be done about it in the moment. Of course that differs from a future threat, I’m not referring to that. But if there is a continued threat, such as an abusive spouse, then often a premeditated physical attack would do more to cause the attacker more problems.
Anger comes from one thing: perceived threat. By reevaluating threats, and over all safety, anger can be mitigated easily.