A society must kill a murderer. If a nation fails to take revenge of the blood of the innocent victim, then that nation must get it in the neck. As a group you either avenge or God will avenge the blood on you!
Thus war is a great debt collector that settles outstanding accounts.
“So ye shall not pollute the land wherein ye are: for blood it defileth the land: and the land cannot be cleansed of the blood that is shed therein, but by the blood of him that shed it.” Numbers 35:33
“That innocent blood be not shed in thy land, which the LORD thy God giveth thee for an inheritance, and so blood be upon thee. But if any man hate his neighbour, and lie in wait for him, and rise up against him, and smite him mortally that he die, and fleeth into one of these cities: Then the elders of his city shall send and fetch him thence, and deliver him into the hand of the avenger of blood, that he may die. Thine eye shall not pity him, but thou shalt put away the guilt of innocent blood from Israel, that it may go well with thee.” Deut 19:10-13.
“Rejoice, O ye nations, with his people: for he will avenge the blood of his servants, and will render vengeance to his adversaries, and will be merciful unto his land, and to his people.” Deut. 32:43.
“The sword of the LORD is filled with blood, it is made fat with fatness, and with the blood of lambs and goats, with the fat of the kidneys of rams: for the LORD hath a sacrifice in Bozrah, and a great slaughter in the land of Idumea.” Isaiah 34:6.
“Egypt shall be a desolation, and Edom shall be a desolate wilderness, for the violence against the children of Judah, because they have shed innocent blood in their land.” Joel 3:19.