Is this a sign of mental illness or just asshole behavior?

My sister Lena’s friend (they are no longer friends actually) Ashlyn (14) seems to have an inflated ego. She thinks she’s better than everyone else and puts people down to prove it.

She specifically targets Lena because she’s there with her and Lena seems to be her scapegoat when something goes wrong. She takes her anger out on Lena and scolds her when something happens (ex: if someone drops something, she screams “Dammit Lena!”). One time, she went to our house to visit Lena and said “I SWEAR TO GOD LENA. I SWEAR I’LL KILL YOU” but then told me she was joking.

She says several racial slurs and the K-word and such to people as well because “she has the right as a black person”, even though she’s not just saying the N word.

I remember once she tried to “ruin Lena’s life” because Ashlyn apparently had a boyfriend or something but was being romantic with another guy. The first guy eventually found out and broke up with Ashlyn. This made her mad that Lena asked if she was still with the first guy (not knowing he found out and broke up).

I tried mediating something with Lena and Ashlyn in the past, but Ashlyn told me “It’s just the way I am, I’m neurodivergent with mental illnesses.”

So yeah, I’m at least very glad that Ashlyn won’t be coming to our house for a while.