John Calvin Commentary Psalms 78:50

"He made a path for his anger; He spared not their soul from death, But gave their life over to the pestilence," — Psalms 78:50 (ASV)

He made a way to his anger. To remove all excuse from this ungrateful people—who, despite the most evident and striking proofs of God's goodness presented before their eyes, could not be kept in obedience to him—it is repeated here again that the wrath of God overflowed Egypt like an impetuous torrent.

The miracle referred to is the last one that was performed there, when God, by the powerful hand of his angel, slew in one night all the first-born of Egypt. According to a common and familiar way of speaking in the Hebrew language, the first-born are called the beginning, or the first-fruits of strength. Although older people approach death as they grow older, yet because they are, in a way, renewed in their offspring, they can be said to recover their diminished strength. Therefore, the term strength is applied to their children.

And the first-born are called the beginning or the first-fruits of this strength, as I have explained in greater detail on Genesis 49:3. The houses of Egypt are called the tents of Ham, because Mizraim, who gave the name to the country, was the son of Ham (Genesis 10:6). Furthermore, the free love of God toward the posterity of Shem is celebrated here. This love is shown in his preferring them to all the children of Ham, even though they possessed no intrinsic excellence that would make them worthy of such a distinction.