The people of Sdom bequeathed to the English language the sin of sodomy. Apparently whoever introduced Sdom into the English language felt that out of all the evils perpetrated in Sdom this particular evil was what set Sdom apart. Chazal felt otherwise. They didn't identify sexual crime with Sdom. What they did brand as the mark of Sdom was something much less sensational.
Selfishness is a normal trait in people.
At first glance their attitude doesn't seem particularly horrid. They weren't warlike, nor did they set out to take what wasn't theirs. Nevertheless they were wiped out. I think that their extreme selfishness was what led to their destruction. At first their selfishness was turned to outsiders but as time went on life within Sdom itself was poisoned by this vice.