Psalms 136:10

To him that smote Egypt in their firstborn
In a tender part, in the dear part of themselves, in their sons and heirs, and who were to inherit their lands and estates, and perpetuate their names; this was an act of justice for using ill the Lord's firstborn, the people of Israel, slaying their sons, and refusing to let them go, ( Exodus 1:13 Exodus 1:14 Exodus 1:16 Exodus 1:22 ) ( Exodus 4:22 Exodus 4:23 ) ; and yet there was mercy in it, for which thanks were to be given to God;

for his mercy [endureth] for ever;
the Israelites, in a very merciful manner, were distinguished by the blood sprinkled on their door posts, when the destroying angel passed through the land of Egypt to destroy their firstborn; and when they were destroyed, it was owing to the kind providence of God that the Egyptians did not rise as one man to cut off the Israelites in vengeance; and yet not a dog was suffered to move his tongue against them when the dismal cry was made; yea, this was the means of their deliverance, which could not be obtained by all the other plagues; but now they not only bid them go, but were urgent upon them to be gone, ( Exodus 11:5-7 ) ( Exodus 12:22 Exodus 12:23 Exodus 12:29-33 ) .