'Why do we have to kill,' Naegi thought to himself. Death should be a foreign concept to everyone at Hope's Peak. How do they kill each other with the slightest motivation? Why do they have to live in fear of each other? They say they all trust each other, but was that really just a farce? Did Naegi really trust any of them? Togami talks of how he doesn't mind the mutual killing. Fukawa is Genocider Syo. He can't even read Kirigiri. What reasons does he have to trust them?
Kuwata killed Maizono, the only person he felt he could trust, and even he had killed her due to the fact that she tried to kill him and blame it on Naegi. Chihiro trusted Mondo with his secret and asked for help, but all he did to help was to kill Chihiro to keep his own secret. Celestia tricked Yamada into killing Ishimaru for "stealing" Alter Ego, and she later betrayed him by killing him and framing Hagakure.
They all see these acts of betrayal and killing one after another, yet they still stupidly agree to meet in the cafeteria every morning? How do they all still agree to stay in their own room after 10 PM every night? They're all stupid. Stupid and helpless and blindly trusting one another. But what else can they do?