Hm-m-m-m . . . you know, Robbie, it has been a few months since we watched it. What I remember is that I thought Jimmy Stewart's character was kind of weak in some places, like when the bad guys got the drop on him and his workers, then ended up burning their wagons. But he eventually did succeed by the end of the movie.
As to your question, I honestly don't remember the details to be able to answer, but I think Dave influenced the Jack Elam character to kill Jimmy Stewart's character.
Chester 
