Genesis 19:17

And it was that as they led them without, one of them returned into Sedom, to destroy it; and one remained with Lot, and said to him, Be merciful to your life; look not behind you, and stand not in all the plain; to the mountain escape, or you perish.

LOOK NOT BEHIND THEE, NEITHER STAY IN ALL THE PLAIN. The purport of the verse is to state: “Do not stay in all the Plain and do not look behind you after you have been saved.” For as long as Lot had not reached the mountain, disaster would not overtake the inhabitants of Sodom. It is for this reason that Lot said, “And I cannot escape to the mountain, lest the evil overtake me when I am in the Plain since you will not extend the time for me more than a little, as you have said, Haste thee, escape thither.”
Rabbi Abraham ibn Ezra said, “Look not behind thee, i.e., ‘thou and all who belong to thee.’ And similarly is the verse, Thou shalt not eat of it.”
But what need is there for this interpretation? The punishment here was not because they would violate the warning of the angel by looking at them. Instead, the angel merely warned them on his own that punishment would overtake them for such a glance, and he warned Lot because of his merit, and all who listened and took warning saved their lives.
Now as to the significance of the prohibition of looking, Rashi said: “You sinned with them but art saved through the merit of Abraham. You are not permitted to see their doom.”
There is yet another matter. Looking upon the atmosphere of a plague and all contagious diseases is very harmful, and they may cleave to him. Even the thought of them is harmful. Therefore, the leper is isolated and dwells alone. Similarly, those who have been bitten by mad animals such as a mad dog and other animals besides, when they look into the water or any mirror, they behold in them the likeness of the offender, and as a result of this, they did just as the Rabbis have said in Tractate Yoma, and as the students of nature have mentioned. It was for this reason that Lot’s wife turned into a pillar of salt for the plague entered her mind when she saw the brimstone and salt are mentioned (Verse 24). Salt however is mentioned in Deuteronomy 29:22, in connection with the overthrow of Sodom. which descended upon them from heaven, and it cleaved to her.
I am inclined to say that when G-d destroyed these cities the destroying angel stood between the earth and heaven, appearing in a flame of fire, as did the destroying angel whom David saw. Therefore, he prohibited them from looking.
In Pirkei d’Rabbi Eliezer there is a similar text: “The angels said to them, ‘Do not look behind you since the Divine Presence of the Holy One, blessed be He, has descended to rain brimstone and fire upon Sodom and Gomorrah.’ The compassion of Edis, Lot’s wife, welled up for her married daughters who were in Sodom, and she looked behind her to see if they were following her. She thereupon saw the back of the Divine Presence, and she became a pillar of salt.”

אל תביט אחריך, first of all on account of their going to experience the anguish of seeing the destruction of their city; secondly, in order not to afford them an opportunity to see the angels performing their destructive work as angels when there was no need for them to witness this. Watching divine beings in action and being saved by such spectacles requires Divine intervention and should therefore be avoided. Compare the awe with which Manoach comments on what he has seen in Judges 13,22, as well as Yaakov’s reaction in Genesis 32,31 after his wrestling match with the angel of Esau.

אל תביט אחריך, the evil would catch up with you as soon as you interrupt your march away from it. This is precisely what happened to his wife when she ignored the instructions and looked behind. (verse 26)

ויהי כהוציאם, while they were in the process of removing them from the city of Sodom they urged them to hurry in order to be saved;

ויאמר, the angel whose task it was to save him, even though there were two angels that worked together.

אל תביט אחריך, they said that to him in order that he should not thereby create a delay. He was afraid that Lot would want to ascertain if the city behind him was really being destroyed.

בכל הככר, in the entire valley, seeing that all the towns in that valley were being turned upside down.

פן תספה, “lest you will be destroyed with the people of the city if you stand still. I have no authority to save you from the destruction once the process has commenced. The destruction will occur instantaneously and simultaneously in al the towns slated for this.” (compare Lamentations 4,6 on the expression ההפוכה כמו רגע).