Or HaChaim on Deuteronomy 7:16/אור החיים על דברים ז׳:ט״ז

1 ואכלת את כל העמים, "You shall consume all these nations, etc." This is a positive commandment. Moses continues that this commandment applies to the nations whom G'd delivers into our hands (the seven Canaanite tribes). There is a clear implication that if the Israelites were to be tardy in carrying out this commandment this would be tantamount to despising a divine gift.

2לא תחוס עינך, "do not display pity, etc." This is what Solomon taught us in Proverbs 12,10: "showing compassion to the wicked is cruelty." Contrary to the common belief that the display of pity is a virtue, the Torah teaches that this is not only not necessarily so, but it can even be a negative virtue. Perhaps the Torah had to warn us about this seeing that G'd blesses Israel with an abundance of children. We have also been taught in Sanhedrin 36 that fathers are apt to show more pity than men who have no children and that as a result men who have no children cannot be appointed to the Supreme Court as they lack a measure of that virtue. To make certain that pity would not be shown to the people whom G'd wants to destroy the Torah had to legislate this in our verse.

3Our verse continues: "and so that you will not serve their deities" (if you allow these people to survive in your country) to warn you that as a result of your misplaced pity you will "shoot yourself in the leg," as their idols will be a snare unto you, eventually leading to your destruction instead of their death. Your pity will be counter-productive.