I'm very interested! :D I honestly was a little disappointed when I realized this game hasn't been developed yet... I was ready for the experience lol.
So what kind of game would this be... Visual Novel, RPG, Platformer? I can expand on some things from there. If it's a story-driven game, is there a message or two you want to get across to the player? That's another great place to start, building everything off of the morals of the story.
I'm interested in this girl character, and how you put "Mentally ill" in quotes... that's implying something else to me lol. Maybe she's just too sheltered because of her homeschooled life, and she's only acting out because she's misunderstood? And everyone thinks she's just "Mentally ill", but really it's psychological damage and/or trauma? That could lead to constant outbursts (but imagining killing people to "feel better" does actually sound pretty mentally ill)
Maybe the friends annoy her at first, and she even tries to kill some of them, but the way they react and treat her changes her as a person? Or not... maybe according to the player's choices, she just straight up murders them all.
Either way I'm super interested, and I'd be glad to even help out here and there if you'd like. I'm a budding artist and composer for indie games... just putting that out there ;)