Charles Ellicott Commentary Psalms 136:10

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

For his mercy.— Here the refrain, after the mention of the destruction of the Egyptian first-born, and subsequently after that of war and slaughter, sounds harsh to Christian ears. But the word mercy (khesed) in the Hebrew motto implies distinctly covenant grace, that special favor of Jehovah in which the heathen did not share, and which was often most notably shown in their destruction.