[duckman]

[ QUOTE ]
Why the ending works

Up until his initial encounter with Evans, Wade has never met a decent human being. His parents were trash and even the "respectable" people of the time could be bought or had their own demons. He treats people like animals because in his mind they are. He spent days reading the bible as a child and had never met anyone that resembled the characters within. He finally meets someone that resembled a decent human being and spends the entirety of the movie trying to find the "animal" within him. The scene in the hotel in which the whole town becomes mercenaries and the sheriffs flee leaving Evans and wade behind is a microcosm of his experiences with people. When Evans confesses to wade that he isnt doing this for himself, but for the sake of his son and his family, he realizes that he has finally met a good man and is compelled to get on the train. He killed his crew because they murdered the only honest man he ever met.

[/ QUOTE ]
Also, he killed the crew to finish the job his father started (so he could get on the train) and show the boy that he was capable of betraying even men most loyal to him and to give the boy an unromanticied view of who he really is. The boy then struggles with the choice of becoming Ben Wade by killing him or going back to the farm and being like his dad.