You don’t believe in genetic mental illness? That one can be born with a sickness in the brain?
You don’t have to believe everyone on the internet, I can only offer you my slice of experience. Nothing wrong happened to my sibling. It was a child who actively tried to hurt people and kill stuff barely after learning to walk. It scared everyone for a while but medication and therapy helped turn they into a stable and functional adult. My sibling is also pretty open about it, at least with me.