Apparently,
Deuteronomy 7 (specifically 7:2) is taken to mean that God supports killing in war.
7:2 And when the LORD thy God shall deliver them before thee; thou shalt smite them, [and] utterly destroy them; thou shalt make no covenant with them, nor shew mercy unto them:
7:5 But thus shall ye deal with them; ye shall destroy their altars, and break down their images, and cut down their groves, and burn their graven images with fire.
7:6 For thou [art] an holy people unto the LORD thy God: the LORD thy God hath chosen thee to be a special people unto himself, above all people that [are] upon the face of the earth.