John Gill Commentary Psalms 136:18

"And slew famous kings; For his lovingkindness [endureth] for ever:" — Psalms 136:18 (ASV)

And slew famous kings
Famous for their power and strength, their courage and valour who were not only smitten and discomfited, but slain in battle;

for his mercy [endures] for ever ;
towards his own people, for whose sake these kings were slain.