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In his heart, Subaru loves Seishirou deeply but can never bring himself to spell it out loud. Any discomfort that may have arisen from the nature of their relationship went out the window as Subaru grows to believe that the older man really loves and cares for him. When Seishirou saw a maniacal woman charging at Subaru with a knife, he dived to save the boy and had his right eye gouged out. He was immediately rushed off to intensive care, and Subaru blamed himself uncontrollably. He was so convinced that the accident was his fault that his own guilt overwhelmed him. But fate would have it that the minute he admits his love for Seishirou, the man would reveal himself as the Sakurazukamori and declare that he felt nothing for him and could break his arm like a glass cup. So Subaru broke down and Hokuto stepped in.
Not only was Seishirou saying he didn't love him, he was also going to fulfil his promise and kill Subaru. Hokuto was intuitive and knew that the Sakurazukamori would come for his brother sooner or later. Waiting for Seishirou to hunt him down was too much of a risk, so Hokuto decided to intervene and end everything so Seishirou wouldn't prey on Subaru anymore. The two were twins, so it would've been easy for Hokuto to disguise herself as Subaru and die in his place. Dressed in her brother's ceremonial robes, Hokuto went to the cherry tree, knowing that Seishirou would be there. The Sakurazukamori, having recovered from his eye injury, was now back on his two feets, clad in a black trench coat and sporting a glassy, dull-grey artificial eye with a cold smile fixed on his face. Seishirou knew the twins well and obviously saw through her disguise, but killed her nonetheless.
With her dying breath, Hokuto explained that she was doing what's best for the both of them by giving them a chance to start anew. Seishirou may be a cold-blooded assasin, but he's human after all, and all humans deserve a chance in love no matter how black or impenetrable their heart may be, because love is not a sin. Hokuto happily sacrificed herself and did what she thought was best for everybody, only her plans were to backfire and cause Subaru to spiral further into depression and trauma. Being told by the man he loved that his feelings weren't return was probably bad enough, but killing Hokuto was adding insult to the injury and having a loved one torn away from one's life so suddenly and in such a violent manner was simply unbearable. But not even the death of his beloved sister could convince Subaru that he'd stopped loving Seishirou...