It's interesting that young Joey looks up to Shane, but Shane respects Joe who might be the unsung hero of this film, deftly played by Van Heflin.
Although Shane has a skill, it mainly wreaks havoc and death. Joe is a farmer who brings forth life and nurtures it. Shane would gladly trade his abilites for Joe's, but could never change his nature or the fact that one day his past might catch up with him (as evidenced early in the film by reaching for his gun whenever he hears a noise). Instead, he uses his killing ability to save those he loves. As he tells Marion "A gun is a tool and no better or worse than the man who wears it." So too does Shane become a tool, like a wild force of nature, cleansing the land so that others might start anew.
One interesting thing to note in the film is that Joey (young Brandon de Wilde) is the catalyst for most of Shane's actions. Shane stays because Joey personally asks him to. When he goes into town an suffers humilation at the hands of Rykers men, he's not bothered by others points of view, but it hurts him to have Joey think less of him. He returns to fight and even Joey tries to get him to leave, but Shane says to him "You wouldn't want me to run away would you?"
By the mid-point of the film it's the collective belief of all the homesteaders that drives him on. Marion has feelings for Shane (although she would never leave Joe), and tells young Joey that he shouldn't like Shane too much because one day he will be moving on and Joey will be hurt. Nice words, but is she talking about Joey or herself?
At the climax when Shane rides into town to face the Rykers, she asks if it is for her that he is doing all this. Shane responds "For you and Joe, little Joey, and everyone in the valley". Shane loves his adopted family, but it's the very people he loves that drive him to a showdown with the Rykers, and utimately isolationism.