The Hunters are birds in a cage; sacrifices offered up by the ignorant. There is no escape. They know what is coming but the poor fools who have trapped them have no idea of the shit storm yet to arrive.
A laughing demon comes by and wounds Dean for the pure joy of having the Winchester brothers in a cage. Demons mock the Laws of War. Jus in Bello would keep the caged birds safe. It would deny Lilith the right to kill Nancy.
It does not matter. A demon ignores such Laws; they are but the Laws of Men.