Why does Jonas tell Asher not to play “Good Guys and Bad Guys” anymore?

Jonas recognizes that "Good Guys and Bad Guys" is not just a game, but a game of war. He watches the children attacking and pretending to fall on the ground, but when Asher pretends to hit him, he remembers the boy who lay dying on the field of battle, and he remains standing, trying not to cry.

Jonas understands that the others do not understand that their imitation war game is cruel and representative of terrible suffering, but their inability to understand further isolates and frustrates him. The incident shows how their innocence of war makes playing at war possible, and it also highlights the lack of understanding that Asher and Fiona have about certain aspects of life as it used to be.