I cannot do any thing. Since the angel had not only been sent as an avenger to destroy Sodom, but also had received a command for the preservation of Lot; he therefore declares, that he will not do the former act, unless this latter be joined with it; because it is not at the option of the servant to divide those things which God has joined together. I am not, however, dissatisfied with the explanation of some, who suppose the angel to speak in the person of God. For although in appearance the language is harsh, yet there is no absurdity in saying, that God is unable to destroy the reprobate without saving his elect. Nor must we, therefore, deem his power to be limited, when he lays himself under any such necessity; [1] or that anything of his liberty and authority is diminished, when he willingly and freely binds himself. And let us especially remember, that his power is connected by a sacred bond with his grace, and with faith in his promises. Hence it may be truly and properly said, that he can do nothing but what he wills and promises. This is a true and profitable doctrine. There will, however, be less ground of scruple if we refer the passage to the angels; who had a positive commandment, from which it was not lawful for them to abate the smallest portion.

Footnotes

1 - "Dum sibi ipse est necessitas." Literally, "When he is his own necessity."

I cannot do any thing till thou be come thither - So these heavenly messengers had the strictest commission to take care of Lot and his family; and even the purposes of Divine justice could not be accomplished on the rebellious, till this righteous man and his family had escaped from the place. A proof of Abraham's assertion, The Judge of all the earth will do right.
The name of the city was called Zoar - צוער Tsoar, Little, its former name being Bela.

Haste thee, escape thither; for I (l) cannot do any thing till thou be come thither. Therefore the name of the city was called (m) Zoar.
(l) Because God's commandment was to destroy the city and to save Lot.
(m) Which before was called Belah, in (Genesis 14:2).

Haste thee, escape thither,.... Seeing he had granted him his request, he is urgent upon him to be gone, and not to delay upon any account, or make other excuses:
for I cannot do anything till thou be come thither; that is, consistent with the decree of God, that Lot and his family should be delivered and preserved, and with his promise made to him, that he would not overthrow that city; and therefore the catastrophe which would befall all the cities at once could not begin until he was safely arrived there:
therefore the name of the city was called Zoar; in later times, and probably first by Lot, from his use of the word "little", which was his request, which Zoar signifies; it before was called Bela, see Genesis 14:2.

Haste . . . for I cannot do any thing till thou be come thither--The ruin of Sodom was suspended till he was secure. What care God does take of His people (Revelation 7:3)! What a proof of the love which God bore to a good though weak man!

I cannot do any thing till thou be come thither - The very presence of good men in a place helps to keep off judgments. See what care God takes for the preservation of his people!