And show you mercy, and have compassion on you, and multiply you, as He has sworn to your fathers: when you shall listen to the voice of Hashem your G-d, to keep all his commandments which I command you this day, to do that which is right in the eyes of Hashem your G-d...
The Ohr Hachaim explains the need for this Beracha. Why would these people require a special blessing of compassion? They just fulfilled a mitzvah, why did they need a new beracha for compassion?
The Ohr Hachaim explains that Klal Yisroel were commanded to kill out an entire city by the sword. Men, women, children, and animals, are all to be wiped out totally, with no quarter given. When a person does such a thing, he awakens a desire for blood and cruelty. The Ohr Hachaim quoted a group of Arab executioners who would kill people who had been judged for the death penalty. They told him that they really enjoyed their jobs, and experienced great pleasure when separating a person’s head from his torso. Their compassionate spirit was deadened and they became cruel people.
There was always a danger that a person who fulfilled this mitzvah of killing the population of a town, even an evil population who had worshipped idols, would awaken their cruel natures. Hashem promises them that even though they should have become cruel people, this mitzvah would have the opposite effect, and they would become more compassionate people.
Not only will Hashem grant us compassion, He will also treat us with compassion. This is the meaning of the word וְרִּ חַמְךָ - and He will have compassion on you. Hashem treats a person the way he treats others. A person who acts compassionately will see Divine compassion in his life. Someone who acts with cruelty, will not experience this. The possuk now means that Hashem will grant them compassion, and then they will also merit Divine compassion in their lives.
The Torah then says וְהִּרְ בֶךָ - and He will increase your numbers. Even though they had killed out an entire town of people, Klal Yisroel’s numbers would not decrease.
כִּי תִּשְמַע בְקוֹל ה' א לוֹקיךָ - when you shall listen to the voice of Hashem your G-d. This is the reason for these promises from Hashem. If the killing of the people of the town that worshipped idols was a human revenge, one could expect a natural outcome of the people turning cruel and rage-filled. However, seeing as this mass killing is by command of Hashem, guarding all of His mitzvos (as Chazal teach us that judging the people of a city that served idols is akin to following all of the mitzvos), our psyches will not be damaged by killing so many people.
