Moses
Deuteronomy 7:2ESV·traditional attribution

and when the LORD your God gives them over to you, and you defeat them, then you must devote them to complete destruction. You shall make no covenant with them and show no mercy to them.

John Calvin Reformed @genevareformer

2. Thou shalt smite them and utterly destroy them. Those who think that there was cruelty in this command, usurp too great authority in respect to Him who is the judge of all. The objection is specious that the people of God were unreasonably imbued with inhumanity, so that, advancing with murderous atrocity, they should spare neither sex nor age.

Matthew Henry Presbyterian @wholebiblehenry

Here is, I. A very strict caution against all friendship and fellowship with idols and idolaters. Those that are taken into communion with God must have no communication with the unfruitful works of darkness. These things they are charged about for the preventing of this snare now before them. 1. They must show them no mercy, Deu 7:1, Deu 7:2.

Commenting on Deuteronomy 7:1-11

John Gill Reformed Baptist @doctorgill

And when the Lord thy God shall deliver them before thee,.... Into their hands: thou shalt smite them, and utterly destroy them; men, women, and children; which was ordered not merely to make way and room for the people of Israel to inherit their land, but as a punishment for capital crimes they had been guilty of, such as idolatry, incest, murder, &c.