This is less of a review than a brief analysis of the movie's "moral" (stop reading now if you don't want spoilers!) Upon re-watching this old favorite of mine, I was struck by the detail put into scenes about prospecting, particularly one that shows the details of panning for gold. Such details contribute nothing to the plot but are still a nice touch, letting us learn along with the characters about the realities of their new work.
Moreover, whether or not it was intended (and with such a prestige production it may well have been), it serves as a metaphor for how the gold itself reveals the true nature of the protagonists. Just as the water reveals which identical nuggets of rocks are valuable, so too does the entire ordeal of prospecting, as Tim Holt nearly buckles under his greed but manages to keep himself mostly morally pure- unlike Bogart, his foil.
Bogart as Fred C. Dobbs is not a monster; he is just a man who does a worse job resisting temptation than he imagined he would. Before prospecting, he was a loveable rogue, his main flaw seeming to be a lack of ambition, who will fight a man for what he is owed but take no more than his due. By the end he is a paranoid would-be-murderer and chronic betrayer, seeing the treachery in his own heart reflected in the eyes of his former friends.
The novel upon which the film was based is said to have been written as an anti-capitalist tract about how money destroys men's souls. As the film holds up today, the message appears to be a bit different. After all, Walter Huston and Tim Holt come out basically decent; only Bogart could be said to be "destroyed" by it, and even that couldn't have happened if he didn't have the hidden potential to be destroyed.
In Treasure of the Sierra Madre, gold serves not as a device to corrupt men, but a means of revealing their true nature. Those who are pure will stay pure. Those who never had the opportunity to be corrupt before will be presented with a chance- and may, like Bogart, discover a nastiness they didn't know they had.