Albert Barnes 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 first-born - (Exodus 12:29). That is, he struck them down, or destroyed them, by his own direct power.

For his mercy ... - It was in mercy to his people. It was the means of their deliverance from bondage, for the Egyptians would not otherwise have permitted them to depart. By all the results of their deliverance, both to themselves and to mankind, the act was seen to be an act of mercy to the world. It was better for mankind that the Hebrews should be delivered, even at this sacrifice, than it would have been that they should not be brought into the promised land.