My current WiP short is a psychological horror that revolves around parents trying to deal with a child that suffers from an illness that at the time the story takes place had not been formally recognized. The parents do the best they can but the lack of information about the child's condition means they don't know how to deal with it and things spiral out of control. I'm doing what I can to portray the illness accurately and to separate the illness from the character but I still worry about falling into the 'violent mentally ill person' trope. I also intend to resolve this, handling the character differently in another story as this one is merely supposed to be backstory for that project, but that still leaves my first project having to deal with it. I doubt either of these stories will see the light of day but I figured I'd seek advice before I make any bad habits as I have another idea for a violent character with mental illness (although in this case they're the protagonist and it's worth noting that in neither case is the illness their character flaw or the cause of violence).
EDIT: To add some clarification, the child had a latent violent capacity before they were symptomatic. The role the unknown illness plays is to complicate the relationship between parent and child in a way that the parents are unequipped to resolve so that the child's ire is directed toward them. The perspective of the parents is learning the kind of child they raised while failing to properly de-escalate.