The Sword of Doom
(1966)

Tatsuya Nakadai is an amoral samurai who agrees to throw a match in exchange for sex with his opponents wife.  His opponent finds out about it and dies in a effort to kill Nakadai. Nakadai then kills 14 swordsmen on the way out of town!

Nakadai takes up with the widow and joins the Shinsengumi.  One night the Shinsengumi goes out to perform an assassination and tries to kill the local fencing instructor by mistake.  The fencing instructor, Toshiro Mifune, kills over two dozen and Nakadai begins to doubt his ability.  Later Nakadai's wife tries to kill him in his sleep and he murders her in the snow.

Finally, Nakadai snaps in a courtesan house and believes that he sees the ghosts of his victims.  He fights anyone and everyone in seven minutes of pure carnage.  The movie ends with a freeze frame of his face revealing his insanity.

Thanks Don P!