There was this one girl in high school my senior year (that's when i was made aware of her, supposedly she's always been like this), who was like this but with her maturity level perpetually stuck in middle school. She's the kind of person who can't hold onto a friend for more than a month (or SO for more than a week, no joke), because she would flip her shit on her 'friends' for the dumbest fucking reasons (such as using the word 'crazy' to describe literally anything). Of course, she would have the 'you know what you did' attitude the entire time and often refuse to even talk to the person who would usually be made aware of her shit once they talked to someone about it. I should probably mention that she targeted new students/freshmen/socially awkward people, which is fucked up in it's own right. Anyways, the scummiest thing she did that i know of is that she 'came out' as trans like a week after a new trans guy came to our school, and she observed that people accepted him and he had no trouble making friends, so she decided to jump on the bandwagon. (neglecting to observe that people liked him for his personality, not his gender identity). She lasted for like two months.