What is it to act for an undefeated reason? I'm not interested in the analysis of acting for a reason (defeated or otherwise). It seems that there's potentially two ways of understanding the claim that we ought always act for an undefeated reason. Suppose R1 and R2 are reasons for X-ing. Suppose R3 is a reason for Y-ing. Suppose no one can X and Y. Suppose also that while R1 defeats R3, R2 could not have defeated R3 if the only reasons that had any bearing on whether to X or Y were R2 and R3. On the first reading, someone who X's from either R1 or R2 acts for undefeated reason. On the second, only someone who X's from R1 acts for an undefeated reason.
Here's a case where the differences between these two ways of talking about acting for an undefeated reason. Spike kills a demon that was about to attack Willow, and while he does what is right (in a sense), we might suppose that he acted from the motive of malice. He didn't care about saving Willow, he wanted an excuse to kill. The reason for which he acted could not defeat much of any reason, but on one reading he acted for an undefeated reason because the potential defeating considerations were themselves 'taken out' by the good reasons there were for Spike to act.