Deuteronomy 20:16-18

But with the towns of the people that GOD, your God, is giving you as an inheritance, it’s different: don’t leave anyone alive. Consign them to holy destruction: the Hittites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites, obeying the command of GOD, your God. This is so there won’t be any of them left to teach you to practice the abominations that they engage in with their gods and you end up sinning against GOD, your God.