Charles H. Spurgeon on Psalms 135:8

Who smote the firstborn of Egypt, both of man and beast. Herein the Lord is to be praised; for this deadly smiting was an act of justice against Egypt, and of love to Israel. But what a blow it was! All the firstborn slain in a moment! How it must have horrified the nation, and cowed the boldest enemies of Israel! Beasts because of their relationship to man as domestic animals are in many ways made to suffer with him. The firstborn of beasts must die as well as the firstborn of their owners, for the blow was meant to astound and overwhelm, and it accomplished its purpose. The firstborn of God had been sorely smitten, and they were set free by the Lord's meting out to their oppressors the like treatment.