Catharsis Factor: After committing so many heinous deeds throughout the film's runtime, seeing Naritsugu Matsudaira finally killed by being Impaled with Extreme Prejudice and then beheaded by Shinzaemon is nothing short of satisfying.
Complete Monster: Lord Naritsugu Matsudaira is the Shogun's half brother, and a firm believer that his connections will maintain his impunity. His very first scene has him raping the daughter-in-law of a lord who's hosting him for the night, and then killing her husband for good measure. This isn't an isolated incident, either. Naritsugu has been doing this for so long, he threatens the very stability of the nation. Later, we see him tie up a family for target practice—including a little girl—and hear how he massacred a peasant revolt, taking the leader's daughter, after cutting out her tongue and chopping off her limbs, for his use as a Sex Slave. His only reaction to seeing the titular assassins mow down his loyal soldiers who die in defense of him is to contemplate how amazing the era of war must have been while deciding to return to such days when he becomes Shogun.
Love to Hate: Lord Naritsugu is an absolutely horrific monster, but he's generally seen as a very good villain thanks to Goro Inagaki's eerie performance as well as his unique characterization as a Blood Knight whose love of war is largely due to his lack of experience with the grisly reality of it.
Shimada Shinzaemon is an older samurai seeking a place to die. Secretly contracted to assassinate the twisted Lord Matsudaira Naritsugu, Shinzaemon rejoices as a chance for such a worthy end and to destroy such a great evil. Struck at Naritsugu's monstrous cruelty, Shinzaemon recruits twelve other warriors, knowing it will mean their deaths, while plotting the defeat of Naritsugu's 200-man strong guard. Shinzaemon converts an entire town into a series of death traps so that the thirteen may triumph, ultimately cutting down countless good men to reach Naritsugu and his honorable guard Hanbei. Shinzaemon ultimately slays Hanbei and allows Naritsugu to mortally wound him so he might finish the sadistic lord himself.
Koyata Kiga is a mysterious trickster who joins the twelve samurai conscripted to kill Naritsugu. Aiding the samurai in journeying through the forest, Koyata proves a cunning fighter as he lays traps for Naritsugu's forces and picks off many of the enemy samurai with slings and rocks. Seemingly killed by Naritsugu himself, the likely supernatural Koyata cheerfully returns after the battle finishes, declaring his time with the samurai fun before departing to reunite with his beloved Upashi.
Moral Event Horizon: Nartisugu pole-vaults over it during his first appearance and keeps on plummeting further down with every second he spends on screen. Trying to actually list his misdeeds would result in a full summary of the film.
Nightmare Fuel: In the first act, we meet a young woman whose father was the leader of a revolt against Lord Naritsugu. To punish her father's act, Naritsugu killed her entire family, hacked off her limbs and tongue, and used her as a living sex doll. This atrocity is what finally drives Shinzaemon the accept the mission to assassinate the depraved lord.
Signature Scene: Hirayama moving down a street of katana, drawing them, and wielding them in battle is probably the film's most iconic bit of imagery.
Too Cool to Live: Subverted with Kiga and Shinzaemon's nephew, but played straight with just about everyone else in the group.
Values Dissonance: Japan doesn't have a problem with non-sexual nudity or children being naked. There's a long shot of a naked little boy, about 6, peeing in a ditch. This is a funny gag to Japanese Cinema (a similar joke would be picking your nose in the West) but it can be jarring to foreign audiences to whom "naked children" means "Pedo Hunt".
