Why do guys blackmail girls a lot?

I mean to clarify this question. I use to be extremely stupid and shit anyway. So this one time I was dating this guy and I trusted him with one my secrets and it was a major secret and then he started blackmailing me a lot and I ended up self harming and some stuff because of all the things he'd say and want. Now my friend is getting blackmailed from this guy because they broke up or something. She hasn't told me the whole story and I'm worried.
So answer this question please. Why do guys blackmail girls a lot? Definitely if they know it will hurt the girl more.
Why do guys blackmail girls a lot?
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