John Gill Commentary Psalms 136:10

"To him that smote Egypt in their first-born; For his lovingkindness [endureth] for ever;" — Psalms 136:10 (ASV)

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 1:13Exodus 1:14Exodus 1:16Exodus 1:22) (Exodus 4:22Exodus 4:23) ; and yet there was mercy in it, for which thanks were to be given to God;

for his mercy [endures] 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 12:22Exodus 12:23Exodus 12:29–33) .