Albert Barnes on 2 Peter 2:6

And turning the cities of Sodom and Gomorrha into ashes. Gen 19:24,25. This is a third example to demonstrate that God will punish the wicked. Compare Jude 1:7. The word here rendered "turning into ashes," (τεφρωσας,) occurs nowhere else in the New Testament. It is from τεφρα, (ashes,) and means to reduce to ashes, and then to consume or destroy.

Condemned them with an overthrow. By the fact of their being overthrown, he showed that they were to be condemned, or that he disapproved their conduct. Their calamity came expressly on account of their enormous sins; as it is frequently the case now that the awful judgments that come upon the licentious and the intemperate, are as plain a proof of the Divine disapprobation as were the calamities that came upon Sodom and Gomorrah.

Making them an ensample, etc. That is, they were a demonstration that God disapproved of the crimes for which they were punished, and would disapprove of the same crimes in every age and in every land. The punishment of one wicked man or people always becomes a warning to all others.

(b) "cities of Sodom and Gomorrha" Gen 19:24,25. (c) "making them" De 29:23 (*) "ensample" "Example"