I don't want to respect my gf's mothers boyfriend.
That's weird. Why don't you respect him?
I note how much he differs from my mother's husband
Even if you don't like that person you should still have some version of respect for them. Just because you aren't a fan of them doesn't mean that you have the right to be rude to them.
Yeah I do have the right, I'm going to blame them and their relationship for my gf's unhappiness.
You shouldn't blame them and you should do more to respect them. They will be in your life if you continue your relationship with your girlfriend.