“When the lights go out and there is no longer any reason to fear police,”
Are you talking about a lack of civilization and no reason to fear any sort of official consequences? That’s a bit more than just fearing someone will call the cops.
There are also social stigmas, and being ostracized, and feeling guilt.
We are cooperative animals and evolved to work in groups. Morality is something we came up with so that we can live in big groups, and make cities.
I do not need to believe in any gods to see that an orderly civilization is in my best interests, as well as everyone else’s.
“what will atheists do?”
Live by our existing moral code, same as any other human.
“What will they not do?”
That’s going to depend on the individual, innit?
“What is the rational reason to not take what you want, kill those who try to prevent, and otherwise just do anything that comes to mind?”
As above. Living a moral life is much better for everyone than being as nihilistic as most Christains predict they’d be without god.