Negaigoto (ねがいごと)/theories

Ken felt himself betrayed
Judging by some events and the True End, Ken might thought that he was betrayed. He had a brother named Yu (the guy from Ending 2) and girlfriend/love interest named Tessen. (the girl from Ending 3). For some reason he began to believe that Yu and Tessen were couple, where basically his brother stole his girlfriend/love interest. Seeing them together broke his heart so he locked himself in his room. Also judging by his dream world and some events, Ken has lost will to live and had suicidal thoughts. In True End, he went mad and decided to kill his brother (and maybe Tessen as well) but failed.