Negaigoto (ねがいごと)/theories

Ken felt himself betrayed
Judging by some events and last true end Ken thought that he was betrayed, situation is, in past Ken had a brother Yu (guy from ending 2 ) and girlfriend/love interest Tessen (girl from ending 3), and Ken can misunderstood and decides that Yu and Tessen are couple, basically his brother stole his girlfriend/love interest, after he saw them together it broke his heart and he locked himself in his room, judging by dream world and some events Ken lost will to live and had suicidal thoughts, but in last true end Ken goes mad and decides kill his brother and maybe Tessen as well, but he failed.