Why does this guy act weird after he dumped me?

Anonymous
so after valentines day I guy that I was in a relationship with ended things because he said he wasn't being faithful to me and I didn't deserve it, so then at school he moves on and starts flirting and getting with other girls ( meanwhile it's only been a day since the breakup) so while I'm hanging out with my other friends who he's also friends with he starts getting jealous and tells one of them to stop going for his leftovers , but then proceeds to allow his bestfriend to shoot his shot, not only that but his bestfriend made a bet with one of his other friends to see who can get with me first ( found this part out today, currently has been two weeks since the breakup) and then proceeds to shoot his shot at my bestfriend! can anyone give me advice please?
Why does this guy act weird after he dumped me?
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