Suppose two men agreed they they are going to collaborate on getting a job done. Working together will make the task at hand much easier than if they were to each undertake it alone. The job requires an investment of capital, weeks of planing, time and effort to be put into it in order to be successful. Suddenly, one of the men backs out of the agreement just as all their hard work was going to materialize. Now the other man is left in a situation where he can not finish the job by himself and he has now wasted money, time and hard work. Was it wrong for the man to back out of the agreement so late in the game where by him doing so, it jeopardized the success of the job?
Think about your answer. What is the moral duty to uphold a social agreement? If one agrees upon a "social contract", does one have the obligation to uphold it? Most people would say that both parties have committed themselves into a social contract, while not necessary legally binding, does contain within it, levels of trust and dependency that if broken would be damaging to the parties involved. It is you can say a "socially binding" contract.
Now let's say that the man backed out of the agreement because he suddenly caught a sense of moral contemplation. The job they were doing was to go out and kidnap a young girl for the purposes of raping and killing her. He backed out of the deal because he just couldn't bare to go through with it. Now was he wrong for backing out? Or, was he right for backing out of an agreement to kill someone solely for its pleasure?
The moral idea of honoring one's contractual and social agreements, and of being an honest broker in business clearly has its exceptions. But therefore, the question begs to be asked: Does all morality depend on situational, and relative circumstances? Is, in other words, all morality relative? When Sam Harris outlines the idea of principle, standing firm even when one can find exceptions, he is talking in a way about having rigid morals that do not necessarily have to be absolute in every situation. For example, in chess there is a good principle to adhere to: Don't loose your Queen. But there are situations when sacrificing your Queen is the best strategic option to make, and there are other times where you will have no other choice but to sacrifice your Queen. These exceptions do not have the ability to erode away the solidity of the principle at heart. No one is going to say that because a single exception can be found to the principle of not loosing your Queen in chess, that we must therefore throw out the entire principle or that the principle is erroneous.
One argument theists make about morality absence of god, is that without the absolutism of morality from divine command theory, you cannot have moral principle. My initial example above on the relativism of a moral proposition, shows how morality, while not always absolute in its nature can still have a principle behind it. In principle, I might say, it is morally right to honor those you conduct business with whether contractually or socially, but there are exceptions.
Why is murder wrong?
It is pretty axiomatic that just about every documented society, and every culture, has a basic, core set of morality that it lives by. It's a kind of basic, universal morality. This very general set of ethics, is the by product of socio-biological evolution for us as social primates. Murder for example, was condemned in almost every fashion, at least within the tribe. Historically, there were always justifications for killing the members of other tribes, and for stealing and pillaging whatever they had. In every single advanced social association of people, there exists rules to live by for the insiders. The fact that there are individual exceptions does not take away the principle behind the moral. There will always be sociopaths, and the mentally challenged who do not care, or are not able to reason their way out of murder. One of humanity's greatest challenges was to remove itself from the tribal identities from which it came from, and to learn to embrace one another as all members of essentially the same extended tribe.
Murder is wrong because in principle, it is in our best option to not do so. Sure, I can murder a single individual with no real consequences for humanity as a whole. Perhaps I can do so even to a million people to the same result. But a society that always permits murder with no consequence however, is risking the benefit of being stable. Stability is what allows a society to prosper and advance. In was agriculture for example, that allowed us to cultivate what was once wild, freeing up our ability (and time)to not have to constantly hunt for our next meal. Suddenly, people were free to concentrate on learning from the natural world what they would previously have had no time for. And it was then that humanity began to flourish under the stability that arose. Would therefore, an argument that agriculture was good for humanity sound like a veritable proposition? If so, then saying murder is wrong for humanity holds the same truth and any exception that can be found does not damage the foundation of the underlying principle.