Why was Lot forbidden to turn around and look back?
Rashi: Having sinned alongside the Sodomites, Lot did not deserve to be saved on his own merit, only on the merits of Avraham. Consequently, he was not worthy to witness the death of his compatriots.
Rashbam #1: In order to break with his sons-in-law who were still in Sedom.
Rashbam #2: So as not to slow down his escape.
Rashbam #3: In order not to see the angels at work unnecessarily. 1
Seforno: The plague that struck Sedom was contagious and would catch up with Lot should he delay. 2
Ramban #1: The plague that struck Sedom was contagious, and like all contagious diseases, even looking at it and even thinking about it can be harmful. 3
Ramban #2: When HaSh-m destroyed Sedom, the destructive angel (whose very appearance was devastating) stood between Heaven and earth; therefore it was forbidden, precisely on account of the danger, to look at it. 4
For so we find that Mano'ach thought that he and his wife would die because they saw an angel (Shoftim 13:22). See also Bereishis 32:31.
As indeed happened to Iris (his wife) when she disobeyed this order.
Like for example, someone who has been bitten by a rabid dog and who looks at a cup of water, will see the image of the dog, go crazy and die. Indeed, that is why Lot's wife turned into a pillar of salt - because she saw the sulfur and salt that descended from heaven to destroy Sedom.
In similar vein, the Ramban cites the Pirkei d'Rebbi Eliezer, who says that the Shechinah descended to pour on Sedom and Amorah sulfur and fire. When Iris saw the back of the Shechinah, she turned into a pillar of salt.
Why did the angel instruct Lot to escape to the mountain? What was special about the mountains?
Rashi: Because Avraham lived in the mountains, 1 and once Lot arrived there, he would be safe.
See above (Bereishis 12:8).
It says in 13:18 that Avraham's tents reached Chevron!
Rashi: Avraham's 'estate' was so large that his tents reached as far as Chevron, though he himself resided in the mountains.
QUESTIONS ON RASHI
Rashi writes: "'Flee for your life!' - Be satisfied with saving [your] lives; pay no heed to your money." How is this implied in the verse?
Gur Aryeh: Otherwise, the term "for your lives" would be superfluous.
Rashi writes: "'In the entire basin' (Kikar) - i.e. the Jordan valley." What is Rashi adding?
Gur Aryeh: Not every flat expanse or plain is called a "Kikar," only specifically the Jordan valley.
Rashi writes: "Himalet is an expression of slipping [away]." What is Rashi explaining?
Gur Aryeh: The term "'l'Himalet" usually means 'to flee from a pursuer,' but Lot was not being chased! Rashi explains that it means "slipping away."
