One of the charges that religious people (Christians) throw at atheists is that atheists have no moral code, no conscience. They can do whatever they want when they want because they don't feel that they are accountable to anybody but themselves.
That's not what this topic is about.
Everyone here knows people who have gained an advantage over others in some way by doing things like cheating, lying, or just being a real jerk. This is on every level. Someone may lie on their taxes to get more money back from the government. Someone drives like an ass and causes others to miss traffic lights while they cruise on to their destination. Somebody cheats their way through college and ends up with a fantastic job they didn't earn.
You know these people. You might even be one of them.
Even before I was forced into the Truth by my mother at a young age, she'd pretty much trained my thinking that lying, cheating, stealing, etc are wrong. Period. But that was with the understanding that there was God to answer to or that there was some sort of cosmic karma in the universe. If that is not true, what really is the downside of doing those things? I don't mean being caught or going to prison or wrecking your car or the normal chance of consequence. What I mean is: What if there was 0% chance of being caught, what is then wrong with behaving this way?
If you knew for a fact that you could never be found out when you lied on your taxes, would you do it? If you knew for a fact that you could stab some fellow employees in the back for a better position with no backlash, would you do it? If you cut someone off in traffic just to get an edge and your car was indestructable, would you do it? Society (and laws of physics) have created downsides to engaging in certain activities but if these were far less likely, does that still make them less acceptable?