We need to understand one thing, the arrival of the Serpent People truly signaled the apocalypse. Therefore, Shenzhou was an exception among all the nations in the world, still maintaining organized government bodies, armed forces, and shelters for the survivors.
What Shenzhou could achieve, other countries certainly could not.
Thus, after the initial brief period of chaos at the onset of the apocalypse, once everyone realized the end had truly come, chaos, killing, and looting became the mainstream of the post-apocalyptic world.
Shenzhou too had its share of 'apocalyptic syndrome', and there were psychopaths committing heinous crimes, but these were sporadic and insignificant. As long as the shelters existed and the vast majority still harbored hope, people would not default to pillaging others for survival.
