So the Torah guarantees us a special gift from Hashem, that if we do that mitzva properly, Hashem will give us in out hearts and extra dose of rachamim, so we will not have our definition as a people undermined.
In this case, as heinous as the crime that was committed, and as much as we know that HKBH will judge properly, and we expect that means things will not go well for Mr. Aron, we must still be careful not to become achzarim ourselves, all in the name of vengeance against a horrible murderer.