leave him
no person is under an obligation to allow themself to be hurt because someone else is unable to take responsibility for their own actions
the chances of him going through with it are very very low. He’s saying that thing because he knows she’s too kind to leave for it. more than likely he’ll just keep messaging her online or the like similar threats and eventually go away when he realizes she’s serious. he’s not interested in hurting himself, he’s interested in hurting her by fucking up her head
i have no real good advice so my advice is fucking fight him. or something. get away. honestly people like that dont deserve your emotional labour because they will drain you fucking dry