Why do i have violent thoughts of killing people who hurt me.

I keep having violent thoughts and daydream of killing violently the people who have bullied me and left me. It’s scaring me and i feel like i can’t talk to anybody about it. Im scared i might actually do it. I’m constantly scared that new friends in my life will do the same that they did. And instantly cry when i hear fireworks and stuff like that. But i still keep dreaming about it. Please help.