Character Obsession

Hi so I have my best friend I know her my whole life but she is maybe weird?
I mean she always was cruel but she never choked me and tired to cut my back with a knife before-
She didin't care that I didin't wanted that ...she did it anyways..What is wrong with her? Also she came up with idea buying an pet and boiling it alive.

Also I think she thinks of me like a toy but I still love her she did a lot for me too! She always tiries to defense me..And she tells me that she loves me but sometimes she is really nasty even to me..
And I think only I know her from this side.Like she is acting normal with everyone ( even if she talks behind their back to me that they mean nothing to her at all but she hangs out with them anyways cuz it's fun)

What do you think what is wrong with her? What I should do? Is she some kind of psychopath? I'm scared