“But against all the Children of Israel, no dog shall whet its tongue, against neither man nor beast, so that you shall know that Hashem will have differentiated between Egypt and Israel” (11:7). How does the silence of the dogs tell us that “Hashem has differentiated between Egypt and Israel?”
Chazal say on Bava Kamma 60b that when the Angel of Death strikes, the dogs cry out. Yet on the night of the tenth plague, no dog even “whet its tongue!” Evidently, Hashem Himself, not the Angel of Death, killed the Egyptian firstborns.
Hashem said that He Himself would kill the firstborns without a messenger. It is said that the best way for a person to die is to have Hashem Himself remove his soul without any intermediaries. We see that despite what the Egyptians did to Bnei Yisrael, Hashem still gave the firstborns the best death that could be given. We should remember this as an example of the great mercy people should have for their enemies. Just as Hashem gave the firstborns a merciful death, we should have mercy on our own enemies.
-Adapted from a Dvar Torah by Chatam Sofer