If Genta did strike Juuzo while he was down, not only would it have been out of character, but he would also have gone against his own morality, therein violating one of the main ideals of Bushido, the code that samurai are meant to live by.
If they all acted on a whim and they didn't follow the principles of Bushido, what kind of samurai would they be? The would be nothing more than your average group of colorful vigilantes with no respect for any laws or social conventions.