The reference to "Lucifer" in Isaiah 14 is not a later addition but is consistent with the prophecy against the King of Babylon that begins the chapter. The figure of the archangel Lucifer as such does not appear in the Old Testament. The Hebrew word הֵילֵ֣ל (hê·lêl) means simply "morning star." It could be associated with a deity such as Babylonian version of Venus, but was not originally understood as referring to an angelic being. Instead, it was a metaphor for the Babylonian King's lofty opinion of himself and his assumption of divine authority. So rather than "switching to Lucifer," the prophet is employing an analogy in which the Babylonian ruler is compared to the brightest star in the sky, which will be brought down the Sheol. It continues the theme introduced earlier in the chapter, in which the prophet is commanded to:
Take up this taunt against the king of Babylon: “How the oppressor has
ceased, the insolent fury has ceased. The Lord has broken the staff
of the wicked the scepter of rulers...‘You too have become as weak as
we!
You have become like us!’ 11 Your pomp is brought down to Sheol,
the sound of your harps; maggots are the bed beneath you,
and worms are your covering."
Another prophet, Ezekiel, used a similar literary device, comparing the
king of Egypt to a towering cedar tree:
I will make the nations quake at the sound of its fall, when I cast it
down to Sheol with those who go down to the Pit; and all the trees of
Eden, the choice and best of Lebanon, all that drink water, will be
comforted in the nether world. 17 They also shall go down to Sheol
with it, to those who are slain by the sword; yea, those who dwelt
under its shadow among the nations shall perish. 18 Whom are you thus
like in glory and in greatness among the trees of Eden? You shall be
brought down with the trees of Eden to the nether world; you shall lie
among the uncircumcised, with those who are slain by the sword. This
is Pharaoh and all his multitude, says the Lord God. (Ezek. 31:16-18)
Conclusion:
The reference to the morning star (translated as Lucifer) in Isaiah 14 is a metaphor symbolizing the King of Babylon's arrogant presumption of divine authority. It is fully consistent with the opening verses of the chapter, which call for a "taunt" against the Babylonian ruler. Therefore, it need not be considered a later addition. Later theology interpreted the verses about the morning star as a reference to archangel Lucifer, who was not a figure known in the literature of Isaiah's day.
