I think even if society broke down, empathy would stop me from killing opportunistically.
I doubt if I would feel too horrible if I killed in self-defense, but even in that case I'd feel regret that the conflict had to end that way.
If society broke down, new forms of social order will be recreated. Organised militia will be formed to protect the property of those who owns something. Sooner or later this militia will be formed into an army and we will suddenly have a new state with new laws and regulations.
