Can I be mad at my ex for seeing her ex boyfriend?
That's strange. Why does your ex seeing her ex-boyfriend bother you?
I just don't don't think they should get back together.
What your ex does now is none of your business, so if she wants to see her ex-boyfriend then you should just stay out of it. When you break up with someone what they do with their love life should be no concern of yours. I suggest you focus on yourself and forget about her.
It's not like that. It's likely not going to have a good ending, I just feel sad for her.
It is nice that you are concerned about your ex-partner. But you should not interfere in her love life not. It is not your place and she doesn't need you to be involved.