Koran 8:67 It is not for any prophet to have captives until he hath made slaughter in the land. Ye desire the lure of this world and Allah desireth (for you) the Hereafter, and Allah is Mighty, Wise.

Make a slaughter in the land means killing all, right and left. It´s the demonstration of the “peaceful nature of islam” – or a warning to the non-Muslims about what expects them?