What's up with Kat

I haven't seen anyone talk about this yet - but I have tried understanding Kat's motivations behind the kind of deep resentment and anger she seems to harbor constantly under the surface?

I get that she is sick and this might be a part of the stages of grief, but it somehow seems a bit more pointed than just being angry at fate for making her sick, her family for babying her or being very religious. I feel like she is angry at someone in her life.

I am especially thinking of the blood ceremony where it feels like she is angry at someone in particular. Like the verse "for the fear, the harassment and the harm that you did" is repeated multiple times. And she sounds like she really means it. It could be argued that she means Corey - but has he harassed/harmed/scared Kat? You could say he did those things to Swann - but the severity and blood oath seems a bit excessive for that a one time incident. It feels more like something she has been holding in and ruminating on for a long time.

Also, at the end when she gets on the truck bed and starts screaming the lyrics of the song, scaring her friends and upsetting Corey and Dylan. Why do that? Who is she angry at? Corey? Dylan? her parents? Is one of the crowd members significant?