Why do I get so angry when someone hurts my girlfriend?

There are some toxic people connected to her last relationship that always drag her down saying things that she's delusional, insane and will never be happy. She takes this to heart as she was being told this when she was dating her ex by her ex and his friends and was continuously told after they broke up. It had been going on for a while and it has done some damage and it's taking some strain on us.

She has told me that she's left them behind but I feel what they have said to her still lingers around rent free. It's even got to the point where she's now defending them and blaming herself.

This gets my blood boiling very high. Not at her but at them for what they have done to her. I may in my emotional haste directly confronted them and got less then ideal results.

I struggle with understanding social situations and to be honest no offense to anyone I struggle to understand girls and women in general.

I love her dearly and I hate seeing her like this because she doesn't deserve to be miserable like this and no one deserves to be treated like this, it's cruel and inhuman.

I just want to help her and I guess I want to know do I have a right to be angry with those who hurt her and if there's anything I can do to help her.