Friend talks bad about everyone why?

[realistic]

I have this friend, she talks bad about everyone. In front of you, she's very nice, but once you leave, she starts talking shit about you. She talks bad about her brother, sister, mom and dad, and her best friends also. She also exaggerates things and also lies.
So I want to know why she does this.

Here’s a little background of her, she is 22 years old college student, and she smokes marijuana, and sometimes does xanax. She used to be abused by her mother, and has been raped when she was 14. She has been physically abused by her boyfriends in the past. She always moved around a lot in her childhood. And one of her friends told me she used to be socially awkward in school..

She doesn't really have that much friends right now, as she ditches her friends. They will make plans with her and at the end she'll cancel it. For example her friend invited her to Las Vegas, got her hotel room and etc, and the day before she cancels it. When I first met her she would do the same thing with me, I would invite her someplace she would agree and then cancel at the end. So I stopped inviting her anywhere.

I am just curious why she does this, I no longer see this person, and I only talk to them a few times a week online.