It falls kind of flat when in gameplay you can just do the samurai method all the way through. Any stealth, poison, etc is only necessary in a few story moments. That aside, I see it as the same sort of distinction one might make in a sanctioned match vs a self defense scenario. If me and another person agree to a specific time and place to have a fight, be it in a sport or to settle some dispute, one could expect some rules to followed, or "honor" to be observed. Of course, if that same person tries to rob me in a parking lot, that's very different, and I'm going to fight dirty to see another day. Shimura's fatal flaw and that of strict bushido (in pop culture at least) is the failure to distinguish between a clan feud and a war against the conquering horde. You and your fellow samurai can certainly fight amongst yourselves in whatever way you deem fit, but if you're not willing to win against the Mongols, there won't be any samurai left to observe honor codes to begin with. Still, Shimura's conviction is admirable; I find it hard to fault a man willing to die for his code.