Dangerous to society?

(Pardon my english)

A friend of mine is kind of violent, at least in her thoughts.

When someone, doesn't matter who or where they are, "bumps" in to her by accident and doesn't apologize, she gets mad. Very mad. She starts to daydream about killing that person, even if she never met her before, and she's never going to meet her again. (she's planning on looking up who this girl is, and go to her house, kill her dog, kill her family, and torture her do death, at least that's what she told me)
For a little tiny thing like bumping in to somebody by accident. And the ways of murdering that person, it could be everything between poking her eyes out, to cutting her arms off and letting her bleed to death.

She also have some kind of paranoia-thoughts. She thinks that somebody's watching her, and that everyone's in on it except for her. Just like "The Truman Show" with Jim Carrey.

She never hurt someone before, and not animals either. But I'm scared that she might, if it goes too far at some point. Her voilent thoughts worries me.

What should I do?
Is this some kind of a mental disorder?