Why is this girl harassing/slandering me?

I was friends with this girl for a month, then we fell out because she was very nasty and undignified. So i politely said you should be happy and me too, so this isn't good for either of us and maybe we should stop talking. She would insult me a lot and did bad things. Then she slandered me with friends who knew better. Then she started chasing my crush, eventually... now. She sends me messages from random numbers, insulting me and saying very nasty things “you’re never going to be good enough” “your a *** weirdo” etc. it can be hurtful and nobody knows what someone is going through. I find it odd, i dont ever want to add to someone’s pain and I've always stayed dignified but i feel like it can look like weakness and such people can become dangerous. But i dont understand why she goes to such great lengths? Why would someone be so psychotic and miserable, why would someone be after ruining you so much? I never did anything to her.
i have considered getting the police involved if she escalates but i need more proof. But i want more insight into why she goes to such extents
Why is this girl harassing/slandering me?
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