Why is there morality?

I'm admittedly a skeptic and have a habit of asking that most annoying question in the arsenal of skeptics: Why?

I think this is the most important question that needs to be settled in the field of ethics before any useful conversation can happen. My reasoning for what I believe is the most reasonable basis of morality is bellow.

Based on our current system of popular belief there is no god, there is no alpha and omega that all existence stands upon, or, if there is, it's existence isn't particularly relevant to the life of man. So assuming there is no entity for morality to flow from then we are left with a social or natural basis for morality. Either morality is naturally a part of us or it exists as a convention.

I lean towards morality being a social convention mixed with certain elements that are naturally innate to every human. There would have been no way for evolution to imprint in us an entire morality, and the differing systems of morality between different times and cultures attests to a constantly shifting morality. However it would make sense that evolution imparted a mechanism in us to prevent us from killing another member of the species. This internal prerogative could be ignored however by a cultures morality, as witnessed by human sacrifice in many early human civilizations.

Is killing ever moral?

Assuming morality is a human convention where social mechanisms serve to modify individuals behaviors for the benefit of society as a whole, then the ethicist must find out if there is an instance of killing that is beneficial to society as a whole. An issue is that there are different kinds of societies and many if not all human societies are not built with the intent to provide for the well being of the general population. For instance feudal society did not exist of the benefit of the serf and bourgeois classes, it served to benefit the feudal lords, and as such the system of morality in these societies reflected this.

Luckily for us, our modern era of liberal democracy is an era which society at least attempts to work for the benefit of its entirety instead of a privileged class or other group.

If morality could be perfectly enforced then the best state of affairs would be if no one killed anyone ever. But unfortunately because this could not possibly be enforced there will be situations where killing occurs outside the bounds of morality.

Then are there any morally legitimate acts of killing? The only possible instance that comes to mind is as a mechanism for punishment (i.e. you killed, you will now be killed for killing). So then is killing the most effective mechanism for punishment/correction? I'm not sure, but maybe other methods of punishment should be explored before resorting to this most extreme punishment.

Is there a significant moral difference between killing and letting die?

This is a hugely open ended question but I will first give the short answer: yes. Then I will give the not-quite-so-shot answer: yes, but not always. And for the long answer I will explore a particular instance of this question, weather leaving a man dying of starvation on the side of the road is the same as killing him.

We first need to establish what the basis of morality is. For the purpose of discussion, and because this system of morality shows a difference between these two acts, let us assume that morality is a convention within society that functions as a series of social mechanisms for the benefit of the society as a whole. Morality is enforced by social sanctions against the transgressor(s). Morality may also be a mechanism within the mind that mirrors the mechanisms of the society around it.

The moral difference in this situation is that ultimately the moral blame cannot be surely pinned to the transgressor of the act. In the case of a man taking the life of a starving man with a gun, it is quite clear that this one man is guilty and the internal and external mechanisms of morality relevant to this man will then act. But the gray ground where maybe hundreds of people walk by a starving man on the side of a city street, who is morally responsible? Are all of these people murderers? And at what point is it too much of a burden to take care of this person that it is not a morally wrong act to not help this man in order to protect ones own interests?