Compare Exodus 12:12, Exodus 12:13 which are closely followed. The only important difference is, the new expression, "The Lord will not suffer the destroyer to come in," which has generally been regarded as implying, that the actual agent in the killing of the first-born was a "destroying angel."
But it is to be noted that elsewhere Jehovah himself is everywhere spoken of as the sole agent; and that in the present passage the word used has the meaning of "destruction" no less than that of "destroyer."
Bishop Lowth's idea of an opposition between God and the destroying angel (Comment on Isaiah 31:1-9.5) is scarcely tenable.