The other reviews pretty much explain what this movie is all about. I would like to add a couple of thoughts.
First, this is probably Alan Ladd's last quality production. The photography and locations are all very good, and the cast is solid. Compare those aspects with Ladd's subsequent films, such as "Man in the Net" and "Guns of the Timberland." Those two are definitely disappointing, not up to the standards of a star who excelled in films such as "This Gun for Hire," "The Blue Dahlia," and "Shane".
Second, the ending undermines the film's impact. Viewers who have seen "The Asphalt Jungle" will attest to the fact that the very grim conclusion of that classic seems inevitable and fitting. In the case of "The Badlanders," I suspect that Ladd himself rejected any such ending (if in fact such had been contemplated).
(By the way, the same can be said for an earlier Ladd film. "Thunder in the East" also has a happy ending that virtually defines the term deus ex machina. Had the principles all been killed in that one, it would have had a tragic quality that would have made it much better.)
"The Badlanders" is a good film (though not a great one) despite the above criticism. Had it appeared right after "Shane," it might have been a major hit. Unfortunately, by 1958 Alan Ladd's personal decline was all too evident. Perhaps it was too late for a Ladd film, even a good one, to break through.