Albert Barnes Commentary Psalms 78:49

"He cast upon them the fierceness of his anger, Wrath, and indignation, and trouble, A band of angels of evil." — Psalms 78:49 (ASV)

He cast upon them the fierceness of his anger ... - This verse is designed to describe the last, and the most dreadful of the plagues that came upon the Egyptians, the slaying of their first-born; and for this reason, there is such an accumulation of expressions: anger – fierce anger – wrath – indignation – trouble. All these expressions are designed to be emphatic; all these things were combined when the first-born were slain. There was no form of affliction that could surpass this, and in this trial, all the expressions of divine displeasure seemed to be exhausted. It was intended that this should be the last of the plagues; it was intended that the nation should be humbled and be made willing for the people of Israel to go.

By sending evil angels among them - There is undoubtedly a reference here to the slaying of the first-born in Egypt (Exodus 11:4–5; Exodus 12:29–30). This work is ascribed to the agency of a destroyer (Exodus 12:23), and the allusion seems to be to a destroying angel, or to an angel employed and commissioned to accomplish such a work (compare 2 Samuel 24:16; 2 Kings 19:35). The idea here is not that the angel himself was evil or wicked, but that he was the messenger of evil or calamity; he was the instrument by which these afflictions were brought upon them.