This makes it lawful for a man to kill a thief, who has not in the least hurt him, nor declared any design upon his life, any farther than, by the use of force, so to get him in his power, as to take away his money, or what he pleases, from him; because using force, where he has no right, to get me into his power, let his pretence be what it will, I have no reason to suppose, that he, who would take away my liberty, would not, when he had me in his power, take away every thing else. And therefore it is lawful for me to treat him as one who has put himself into a state of war with me, i.e. kill him if I can; for to that hazard does he justly expose himself, whoever introduces a state of war, and is aggressor in it.

“Second Treatise of Government” - John Locke