Why does it rain both fire and brimstone (gafris - 19:24) on Sodom surely one or the other was enough? Furthermore, if the whole city is 'turned upside down' (19:25) why was there a need to rain anything down at all?
Rav Moshe Shapiro in his lectures on the parsha asks a similar question. Here is the relevant extract of his analysis.
Mishna in Pirkei Avos says that the one who says "What's yours is yours, and what's mine is mine" is an ordinary person; but some say this was the opinion of the people of Sodom.
The meaning of this, is that the people of Sodom wanted to build a society where people would receive only that to which they have a direct right, for example by earning it. In other words, a society that is based on the pure middas hadin, denying rachamim the opportunity of existence.
This also explains Sodomites' law insisting on causing suffering to people who practice chessed and people to whom such chessed was done - it undermined their philosophical outlook.
Therefore, as what they did was not true din, but an inversion of true din, they get destroyed by Gavriel, the correct manifestation of middas hadin, and the instrument of realization was fire - the symbol of middas hadin.
The Midrash says that the reason why Sodom was punished the way they were was because Sodom was in Israel. In Israel a person is judged to a greater extent. One reason is because living in exile you get an atonement. Also we see from The Ramban that the Avos kept the mitzvot in Israel and Rachel died in order for Yakov not to violate having two sisters in Israel. Therefore Sodom was punished greater for big sins. As opposed to the same big sins outside of Israel where they are taken out through war.