Another interpretation of the ending (SPOILERS)...

The general consensus seems to be that Wade shot Charlie dead at the end because he had grown to admire Dan's tenacity and the strong qualities he was trying to instill in his son. However, Charlie seems to have put Wade's life at risk by offering up $200 to a town full of gung-ho shooters all competing for the demise of his captors.

At the beginning, Wade shoots Tommy after he failed to make sure that the guy was dead in the carriage when he went to get the money. He mentions something about putting everyone's lives at risk by not following the rules, and that being an unforgivable mistake. I don't think Wade or Charlie could have foreseen that any of his captors would refuse to surrender, but as soon as Wade and Dan step out the back door they are being shot at by everyone. I doubt Wade would have been too impressed with the situation that Charlie had inadvertently put him in, regardless of if he could have foreseen it or not.

Also, Wade mentions to Dan that his men are all animals, implying that he holds a low opinion of them and deep down is only using them to suit his own needs. Now that they have put him in a situation that is out-of-control, he sees it as a breach of his rules and they all have to die.

That is my interpretation of the ending, that's not to say that others are wrong, but it's just the way I saw it. Feel free to disagree, but does anyone have any other interpretations of it?

Don't put the devil in the picture, cause' the religious groups won't wanna see it.