Jamieson, Fausset and Brown on Matthew 2:19

19. But when Herod was dead--Miserable Herod! Thou thoughtest thyself safe from a dreaded Rival; but it was He only that was safe from thee; and thou hast not long enjoyed even this fancied security. See on Mt 2:15.

behold, an angel of the Lord appeareth in a dream to Joseph in Egypt--Our translators, somewhat capriciously, render the same expression "the angel of the Lord," Mt 1:20; 2:13; and "an angel of the Lord," as here. As the same angel appears to have been employed on all these high occasions--and most likely he to whom in Luke is given the name of "Gabriel," Lu 1:19, 26--perhaps it should in every instance except the first, be rendered "the angel."