The middle grandson, Sa Sakujun finds the world very boring and seeks to amuse himself without regard for morals. To relieve his boredom, Sakujun became involved with the Satsujinzoku when he was a teenager. Fourteen years ago, he found and recognized the young prince Seien (later Seiran) collapsed in the snow and brought him into the gang. Nine years ago, when his great-uncle Enjun Sa tried to rein in Sakujun’s behavior from a distance, Sakujun covertly retaliated by killing Enjun’s son and daughter-in-law because they were available and vulnerable to him.
When his grandfather Chusho tells him to marry the Kou princess, Sakujun assumes an alias as Lin Senya, a merchant’s son, and arranges to meet Shuurei in person. Although she is twelve years younger than him and has a very different personality, he becomes sufficiently intrigued by her to arrange the murder of his older brother, Sōjun; this places Sakujun as the next in succession to become the Sa clan head, giving him more power to resist the Kou clan if he marries Shuurei against their wishes.
With his typical devious perversity, he eventually allows Shuurei to poison him without her knowledge.
Source: http://saiunkokumonogatari.wikia.com/wiki/Sa_Sakujun
Here is the scene of his death with portuguese subtitles