How can You deal with a psychopathic relative at a legal level?

"deal"? At a "legal level"?

Simply have nothing to do with them. If there is financial business, wrap it up, using an attorney if need be.

If you think they need commital, call your local psychiatric unit and explain the situation as you see it.

If you think that a crime has been committed, then you should call the non-emergency number of the police. If you think the crime (a violent one) is in process of being committed, call the emergency services number and ask for the police.