The 49th prohibition is that we are forbidden from allowing any individual from the seven nations to live. [This is commanded] in order that they not ruin other people and cause them to err by following idolatry.

The source of this prohibition is G‑d's statement1 (exalted be He), "Do not allow a single soul to live."

Killing them is a positive commandment, as we explained in P187. One who had the opportunity to kill one of them, and did not do so, transgresses a prohibition.