Is it okay/normal to say mean things to someone's face because they rejected you?

This may seem like bait, but I'm asking a genuine question. This happened to my girlfriend where she rejected her former best friend (in a nice way…

Definitely not okay. And it depends on your definition of “normal”. Is it common? Somewhat. Going as far as to say “kill yourself” usually isn’t part of it but insulting someone after being rejected isn’t rare. It depends a bit on the age.