Or HaChaim on Deuteronomy 13:18/אור החיים על דברים י״ג:י״ח

1This is the intention of this section here, according to the commandment of a city that is rejected, that this whole city [that rejected G-d] be killed by sword, and even their animals. This act will give rise to the nature of cruelty in the heart of man, as the Ishmaelites told us in the king's article that they have a great desire to kill a person and uproot them. The promise of God is that He'll give them mercy, even though nature will give birth to cruelty. The source of mercy [G-d] will affect the power of mercy [within you] to negate the cruelty that was born by virtue of the act [of killing out the city]. And it says about this city [that was killed out] as long as a man has a cruel nature, G-d will behave with him the same way, that G-d is not merciful [on the cruel-natured man], but compassionate.