Why did Hashem need to "go down" to see what was going on in S'dom?
Rashi #1: To teach the Dayanim not to issue rulings in capital cases before visiting the scene of the crime. 1
Rashi #2 and Targum Yonasan: What Hashem meant was that that He needed to delve into what their behavior will lead to 2 (to examine whether they were truly deserving of extinction or whether they would perhaps relent and receive a smaller punishment 3 ).
Ramban (according to Kabalah): 'I will go down from Midas Rachamim to Midas ha'Din, and see with mercy whether they actually did like the cry that rose to Me from the Midas ha'Din, then they will be destroyed.
Seforno: To delve into their behavior, to determine the extent of their wickedness, and how low they would sink.
Lev Eliyahu (Bereishis p.57): This teaches that Hashem does not run the world according to His knowledge (that Avraham's Tefilah will not help, for there are not even 10 Tzadikim), rather, through cause and effect. He gave to Avraham a chance to pray for them.
Which cries is Hashem referring to in this Pasuk?
Rashi #1 and Ramban (on Pasuk 20): the cries of the oppressed that emanated from the entire city.
Rashi #2, Hadar Zekenim and Targum Yonasan: The cries of a girl (Pleitis, the daughter of Lot) whom they killed in a most cruel manner 1 for giving a poor man some food.
Seforno: What Hashem meant was that He would test them to see whether they would put their inherent evil into practice when Lot would attempt to rescue the angels from their hands -so that the world would know that their punishment was well-deserved.
By smearing her with honey and leaving her to the bees.
What did Hashem mean when He said "Asu, Kalah, ve'Im Lo Eida'ah"?
Rashi and Rashbam: If they continue to rebel (against Hashem - Rashi); If they did indeed behave in this way Hashem will destroy them. 1 Otherwise, He will not destroy them, but commute their sentence to Yisurin (suffering). 2
Ramban (on Pasuk 20): He will see for Himself if they all sinned, and if not, He will 'visit their sin with a staff and their iniquity with plagues". 3
Seforno: He meant that the following incident will demonstrate how ... they would all be guilty; not one person in Sedom would stand up for Lot! 4
Targum Yonasan (See 18:21:2:2). If they have done Teshuvah however, they will be meritorious; it will be as if I did not know about their sins, and they will not be punished.
Moshav Zekenim: If they have no merit, I will finish them off. If not, I will afflict them.
The line in between 'Asu" and "Kalah" indicates that there is a break between the two words (Rashbam).
Hashem was merely indicating that the final judgment had not yet been passed (in which case there was room for Teshuvah and Tefilah).
Why didn't Hashem send a prophet to rebuke Sedom before destroying them, like He did for Ninveh?
Moshav Zekenim: He did! Lot told the people that the men (angels) were sent to rebuke us; they said "va'Yishpot Shafot" (we do not want this).