IMO, attacking a crew member or passenger crosses a line, and it becomes imperative that the attacker be neutralized. Doing so without subjecting oneself to physical harm is also very important while the well being of the attacker is not. It is reasonable to expect that this is a life and death issue and that an appropriate response is justified.
There happens to be an item on every aircraft which is capable of incapacitating an attacker and allowing them to be restrained, if still required, without requiring physical contact by the defender and the item poses no harm to the aircraft either. That item is a fire extinguisher. Just discharge it at the attackers face. End of threat!