Confused?

I found this movie somewhat confusing...why does Zatoichi have sex with that Yakuza lady who never appears again? Why does he come out of nowhere to kill everyone at the end of the film? He didn't seem involved in the conflict at all.

I know this was the last of a very long series of Zatoichi films, but I haven't seen any of them. Is greater familarity with the series lend greate insight? Did that Yakuza lady character appear as a young girl in a much earlier film?

As for Zatoichi's sudden attack, my only idea is that it's a joke in some sense...this is a Zatoichi movie, so Zatoichi has to save the town by massacring dozens of Yakuza...why bother explaining why?