"I hate to break up such a nice romantic scene, but I want you to die." Movie.
"Do you know why this sword has no guard? I never defend, I will incapacitate you with one blow." Movie.
"With that wonderful expression...they beg me to kill them." Manga
From Bijomaru's rantings in the final moments. He tears out his bowels and screams not at sight of blood but the fact that the blood is red and like the color of his 'mother's.' He dies saying 'This is all mother's fault.' The manga implies that Bijomaru like some dysfunctional families also explored in Azumi, was made a monster from the constant abuse and psychological pain he experienced as a child.
He seems to be from a samurai background. Growing in the bushi class. He often look down and is snobbish towards lower working classes as when he referred to the ninja 'Saru' (a.k.a Monkey), a 'lowly' person.
In the film....he vaguely asks the ninja freeing him if he understand the price of more people regardless of innocence being killed for letting him out.
Movie live adaptation
In a highly fictionalized era under the iron fist of Tokugawa reign following a bloody warring period known as Sengoku of Japan, 3 warlords still thirst for bloodshed as well as more wars for personal acquisition of power, nothing is black and white in the world of Azumi...
Yet despite the gray boundaries of morality, the Handsome Fencer still stood as an eccentric and interesting psychopath that both the side of the self claimed 'protagonist' band of child assassin, as well as the brutal militants of Kato Kiyomasa's clan, despise to their visceral soul. Unlike neither the child soldiers and proclaimed 'peacemakers' nor the brutal but cunning warlords bent on national domination, Bijomaru only has one purpose in life. Becoming a literal personification of shinigami and death as a grim reminder to the sane ones...as evident by his favourite kimono, a sober white gown like a bloodless cadaver and an sick, grotesque obession for the meaning of flower as funeral prye items.
The Kato Kiyomasa's ninja's in particular hating his cruel, inhumane habits of playing a defeated warrior by slow physical and emotion torture with a slow death, which sometimes served absolutely no purpose in military strategy other than for Bijomaru's own enjoyment in extremely gory sadistm and his nihilistic fantasy driving him to live to prove it; life is nothing. That everything a man or woman fights for ends in tragedy and meaninglessness, no matter how brave or successful they are. Mogami is remorseless and apathetic towards life itself. Willing to kill a girl just so he could torture a defeated teenage assassin for longer (Slicing him joint by joint in the manga.)
While the movie version reveals little of Bijomaru's background other than a once great samurai turned ronin who went crazy and ended up locked in a dungeon for the sole purpose of keeping in a highly efficient, emotionless killing machne....the manga delves more into Bijomaru's abusive upbringing by his mother and possibly hint that he has a deep sexual disgust by dressing up like a woman in female outfits of the time, wearing long loose hair with eye make ups. It also suggest that his mother was abusive which somehow caused his twisted and vile view of life, which eventually led to him murdering her and striving to murder more woman and mothers to stop it all happening again.
Martial arts and abilites
While not specified in the film...the manga hints to the reader that Bijomaru attained the mastery of a branch of Koryu Iaijutsu school, as most of his offenses are blended with the drawing of the sword and he rarely unsheath his sword in preparation against an opponent in the manga. Preferring to keep the weapon concealed and only draw it at the correct time. The enemy not even noticing the effective attacks.
Not being restricted to style...he is shown to have near perfect achievements in the Japanese Bujutsu's philosophies of "Mushin no Mushin." He was able to tactically predict his oppoent's fighting style by their weapons, body geometry, calculus of certain cuts before the fight begins. Going head to head arrogantly and conceited against the seemingly frail effeminate man is certainly a duel doomed in agony and misery as a the duel in a samurai challenge suggests. The uncanning accuracy allows this murder to cut off the weapon hand and the finer hand of the already very accompolished foe in a beautiful and gruesome fashion.
The former samurai also mastered the philosophy of "Ikken Hamatsu" to the point of being unmatched and completely throwing the best warriors totally out of their comfort zone as Bijomaru fully believes that the best defense is a good offense and taking blows is 'unskillful' while the true ultimate swordsmanship involves in killing and maiming in one blow. That is highly evident as instead of the conventional katana duel sword, Bijomaru's white Japanese sword is a mixture between a sturdy functional katana and that a sword mount storage Shirasaya without a tsuba(sword guard). Without the guard, the storage blade which were never meant to be an actual weapon proved to be useful in the hands of skilled masters e.g. Zatoichi series. The nature of the weapon often tricks opponents into prejudicing moves to attack Bijomaru's arms...a mistake which they pay dearly for. Timing, accuracy and speed is what gifted this crazy maniac a status of being one exceptionally skilled killer who everyone needs...but just hates.
While being an ex-samurai...it is not to say Bijomaru have some stealthy techniques of his own, neither, as he found the hiding master in a stable without much problem and stabbed a yari to initimidate the elder gentlemen so he could have fun killing face to face.
Bijomaru is among one of the best and most refinely skilled assassin in both the movie and manga, rivalling Azumi's master (former general) in a cat and mouse fight involving both swords and other weapons amplified with a vast array of intelligent strategy and improvised tactics. In the movie, he is able to almost gain the advantage and nearly kill the titular protagonist assassin Azumi if not for her double edged sword deflecting certain cuts and her using the environment to her advantage.