Okay maybe managed "perfectly well" isn't true, but managed, yes.
There aren't nearly enough places in secure hospitals to house everyone in the prison system with a formally diagnosed mental illness. Prisons are very quick to refer to secure MH units as soon as the criteria for "unmanageable" within a prison environment is met.
I think prison is the right place for him. He planned and carried out the execution of his father, planned it, obtaining a weapon, loaded and then reloading the gun to carry it out. He nearly killed his mum but was able to exercise self control and not go through with it. From what I can gather, even the defence psychiatric report didn't recommend a hospital order.