How do i tell my ex girlfriend that i went through her facebook without her getting mad at me?

[Dickey Dick]

That sounds like very hard to achieve, pal. But first you tell me, is she the girl who would be really upset about things like that? If the answer is yes, then there's no way around it. Don't tell her. If you feel you cannot stand what you feel about those messages then be brave enough. She is your ex girlfriend after all!
Another approach could be trying to get her say something about those messages or the people who send them w/o her knowing you already know. There must be a way if you really know her, boy.