Jonas has already received the memory of war when the undeclared holiday occurs. With that memory has come understanding of what it means to shoot guns at people, that killing people is not simply a game that you can start and stop at any time but that death is permanent and that being shot is painful.
Because of these insights, Jonas sees the good guys/bad guys game not as a game, but as people inflicting pain and inajury and death upon each other. He attempts to convince the others that this is an awful activity in which no one should choose to take part. However, the others do not have Jonas's perception or understanding. To them, it's a fun game to play with some of their unexpected free time.