Compared with the rest of the story, there is a sudden change in the pronouns from plural to singular. Notice verse 17, "So it came to pass, when they had brought them outside, that he said"—one of the messengers speaks. In verse 18, Lot addresses them and uses the term "lords." Keil & Delitzsch Commentary says, No, "lord" is singular in the Hebrew.
Adonai, "Lord," is the name of God. Is it the name of Melchizedek? Was Melchizedek there to destroy the city? If the messenger was not Melchizedek, why did Lot call him "lord"? Why in verses 21 and 22 does the angel take the authority to himself to destroy the city? He says to Lot, "I cannot do anything until you arrive there," and then verse 24 relates, "The LORD rained down. . . ." Is the LORD the same "lord" who was the "I" of verse 22? An interesting sequence of verses.
Not only does Keil & Delitzsch say this, but the Jewish Publication Society's Tanahk, the King James Version, and the Revised Standard Version all reinforce this idea in various ways.
John W. Ritenbaugh
Faith (Part 4)