Yume Wo. (夢を。)/theories

Father's sin
From events and ending seems to that in Family Massacre guilt is Nagatsuki's father, probably he planed that Nagatsuki kill his mother and himself, for this he make a special tea that Nagatsuki-kun drank, but in fact his plan is backfired at father and Nagatsuki killed both parents, not in his mind.

Friend among the enemies.
Its can be that calm smiling chaser Yujin, who gives to Nagatsuki school uniform was his friend, and when they were friends bullies don't bullied Naga-kun, but fake, obviously Nagatsuki been in Yujin's debt.