I just read a wonderful vort from the Chazon Ish on Rael Levinsohn's new blog:
"'Your eye shall show no pity on them (Deuteronomy 7:16). We learn that Israel acted against its merciful nature [since otherwise this command would not have been necessary]. Even though the seven nations were despicable as a result of their abominations and had declared war on Israel after being asked for peace, the natural pity and forgiveness in the hearts of the people of Israel overcame them, and they acted only because of God's command — not as, heaven forbid, those who take pleasure in war and aggression." (Chazon Ish, Teharot, 299a)
