He prohibited us from keeping someone from the seven nations alive - so that people not isolate themselves with him and he incite them to idolatry. And that is His saying, "you shall not let a soul remain alive" (Deuteronomy 20:16). And their killing is a positive commandment, as we explained in [Positive] Commandment 187. And one who transgresses it and does not kill one of them, when it was possible for him to kill him, has transgressed a negative commandment. (See Parashat Shoftim; Mishneh Torah, Kings and Wars 5.)