Why do so many "nice guys" get pissed off and call you slurs if you reject them? Even if it's in the nicest way possible?

DON'T START ATTACKING ME, I'M NOT SAYING ALL OF THEM DO IT!!!
But the other day I had it happen to me. They called me beautiful and said "he wouldn't treat me like the other garbage I'm into" (probably referencing the rockers I publicly say I crush on. I kindly rejected him. Stating that it wasn't him personally I'm just not in to a relationship. First of all he was like 38 and I'm 15. And he just starting calling me rude things commenting on how no one is gonna want me because I'm fat and ugly and I look like a literal hippo.
Why do some men do this?
Why do so many "nice guys" get pissed off and call you slurs if you reject them? Even if it's in the nicest way possible?
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