I sometimes like to think of ethics as an incentive system that is (generally) stronger than social norms but weaker than laws. Once you reduce it to incentives, it is a matter of comparing incentives. If the incentive to behave ethically is much lower than the incentive to not, then people will pick the better option. That some people seem to be more ethical than others can be viewed as the extent they estimate the incentives from different behaviors (importance placed in the thoughts of others, if they believe some greater power judges them for their actions, etc.).