12 Monkeys, even if I agree that what you said above could be possible, it still comes back to "How did the concept of good (communal living) and evil (every man for himself) come into being?" The fact that we once lived in communities and now live in societies doesn't answer the question of where the concepts of good and evil come from.
The term "sin" has two meanings: 1) doing something wrong and 2) irking god in the process. Toss in the concept of "evil", which is offered up as a noun, a thing, that exists in this world and that has measurable amounts of say in how people behave, and christians give us a one-two punch in the gut as they misrepresent what is actually going on.
There is right and wrong. Not good and evil. There is wrong, not sin. Christians are making human problems worse by pretending that the problem is more complex than it really is.
I could have robbed a bank today. I guess. I'd need a gun and motivation and stuff, but in theory I could have done it. There are no physical controls in my life that would have prevented me from trying to get a couple extra thousand bucks just before Christmas or something. But I am one of the many that agree that robbing banks is wrong.
If I did rob a bank and get away, I would have accomplished the following: a) received ill-gotten gains, b)probably have scared the crap out of at least one teller, c) inconvenienced the police, who already have enough to do, d) inconvenienced a reporter or two who would feel somewhat obliged to write up/do a TV segment on the robbery, e) made myself a wanted man, who would continue to inconvenience the police until I got caught and f) then cost the taxpayers more money via the court process and my incarceration.
By definition, my actions would qualify as "bad", because there was no benefit to anyone but me, if having an extra grand while being hunted by the local police and the FBI is considered a pleasant by-product. Most of us recognize that our society cannot exist by depending on the forced reallocation of resources. We can't get bank employees to provide needed services if they think they will be subject to guns in their face every day at work. We can't afford to get potholes filled if all our police money goes towards catching hundreds of bank robbers a day. So again, society says robbing banks is bad.
I'm not talking evil here. Because the religious have co-opted the term and if I try to use it, the two meanings it has get mixed up. And the term "sin" adds nothing to the table. It is another word for bad that adds the god thing, and that's useless.
So even though I am not one of them thar' god believers, I have no trouble, in this case or most any other that you can think of, in doing the right thing. I'm not even tempted to rob a bank, because I wouldn't feel good about it even if I got away. Because I recognize that stealing is wrong. And stealing has been wrong in many a society never exposed to christianity, so don't go taking all the credit. The Chinese were executing thieves long before christians showed up. American Indians were banishing wrong-doers long before Chris showed up and ruined everything.
All of this is because societies have semi-successfully parsed what is good and what is bad, and gone to the trouble of creating norms of behavior that it does its best to pass on to its members. It isn't a perfect process, but generally it works.
So good and bad are useful concepts, though they can vary from culture to culture. But sin and evil are unnecessary components of the discussion because they add nothing useful, and detract from the issue by injecting unnecessary complications, that are based on serious wrong-thinking.
And yet many theists think that both of those concepts are so important that it is all but impossible to be a decent human being without having them thoroughly ingrained in us. We have had theists here say that if they didn't believe in god, they would not be able to control their desire to rape and murder. Which is patently ridiculous unless they are indeed so brainwashed by religion that their behavior is exclusively controlled by their beliefs. Which is pretty scary.
edit: fixed one of those damn autocorrect problems