The ending is kind of weak, I agree. I understand the logic behind it: Frank (Duryea's character) is trying to drag Kelly (Joan O'Brien) to Del Cobre, where he'll get her money and then probably kill her, assuming they aren't both killed by the Apaches first. Ben (Murphy's character) has the option of either leaving them to their fate or trying to talk Frank down off the ledge, but if it comes down to choosing between Frank and Kelly, the whole "cowboy code" thing requires him to back the woman. And the film's already established that Ben has what you might call an impractical sense of honor, so from that point of view it's not surprising that he does what he does.
The main problem is that Dan Duryea plays the character as kind of a jolly, unmenacing buffoon, so that his death leaves a bad taste in your mouth. If the character came off as more of a twisted, somewhat crazy guy with a few redeeming traits, who REALLY jumps off the deep end once he realizes that the woman is gunning for him, the ending would probably work better. Well, ok, you'd still be left with the question of how a mere cowhand like Ben can outgun a professional like Frank,
and you'd still be pondering the fact that the closing line "He got six black horses. And he slammed the door behind him," sounds like a lot more like Murphy's death IRL than Duryea's.