At that time, the Lord rained down sulphurous fire upon Sodom and Gomorrah and overthrew those cities together with their inhabitants. In Genesis 18:20-33, Abraham engages the Lord in a conversation in which he boldly interceded for the cities of Sodom and Gomorrah. As the two ‘parted’ ways, God promised Abraham to desist from destroying the two cities if God were to find at least ten righteous persons in them. But as we have read in today’s passage, it appears that the inhabitants of the cities were past any redemption. God was not left with any choice but to proceed with his plan of doing away with the two cities. The extent and mode of the destruction meted by God spoke to the gravity of the sins of the two cities. 
The gravity of the sins of Sodom and Gomorrah can be gauged from the treatment that Lot and his visitors (angels of God) were given. Not only was the demand they placed on Lot’s visitors unnatural and unpleasant in the eyes of the Lord, their demand was an affront to an unwritten code on how to treat visitors. Whereas the Mosaic code on the treatment of visitors (aliens and strangers) that is found in the Hebrew Scriptures comes later on, it appears that there were already some unwritten laws that were in use. We see Abraham extending hospitality unknowingly to angels of God that he mistook for travelers (cf. Genesis 18:1ff). A similar behavior is exhibited by Lot to Angels of the Lord whom he also mistakes them for strangers (cf. Genesis 19:1ff). The behavior of the inhabitants of Sodom and Gomorrah towards the Angels of God appears to have gone against this unwritten code of hospitable treatment of visitors. It is a ‘hypothesis’ that is backed by Lot’s reason for offering his two virgin daughters in place of his visitors: ‘ do nothing to these men since this (safety ) is the reason they have come under the shadow of my roof’ (cf. Genesis 19:8). Lot was concerned, not with the unnaturalness of the act they proposed to commit (having carnal knowledge), but rather with the safety of his visitors. Treating strangers and aliens with courtesy is what was expected of the inhabitants of Sodom and Gomorrah. But they revolted against this, thus the reason for their annihilation.