Albert Barnes Commentary Exodus 12:29

"And it came to pass at midnight, that Jehovah smote all the first-born in the land of Egypt, from the first-born of Pharaoh that sat on his throne unto the first-born of the captive that was in the dungeon; and all the first-born of cattle." — Exodus 12:29 (ASV)

This plague is distinctly attributed here and in (Exodus 12:23) to the personal intervention of the Lord; but it is to be observed that although the Lord Himself passed through to smite the Egyptians, He employed the agency of the destroyer (Exodus 12:23), in whom, in accordance with (Hebrews 11:28), all the ancient versions, and most critics, recognize an Angel (compare 2 Kings 19:35; 2 Samuel 24:16).