Ok, here is a super long answer as this was basically the exact question I did for homework:

It must first be said that people do murder in many different circumstances, whether it is an honour killing, capital punishment, war, etc. Therefore, it must be said that nothing prevents murder, but something acts as a deterrent. However, not all murder is considered bad by our societal standards, with the best example being in war. A war veteran, though, after murdering in war, will not necessarily murder at home. Therefore, it must be said that murder is not simply prevented, but is a decision made based on circumstances. By recognizing that murder vs. not-to-murder is a decision, it can be deduced that our reasoning is a part of this.

                Whether or not to murder is a moral decision, as it is a question of what one ought to do. Also, it is a decision that regulates our relationship with others, as morals do (i.e. do not steal, do not kill, do not lie, etc). Therefore, the nature of the relationship between oneself and others must be examined. I know what I am thinking. I do not know what you are thinking. I know how I plan to act. I do not know how you plan to act. This applies vice versa too. This is based on the premise that one can only know himself absolutely, but not know others absolutely. However, I can and do assume what you are thinking and how you plan on acting.

With the case of morals, I can – using reasoning – assume what others would think based on my actions towards them. As we only know ourselves – and cannot know the thoughts of others – the following is our only means of making this assumption. If you insult me, I know a few things: I will be hurt to some degree, I might walk away, I might insult you in return to hurt you equally, or escalate the confrontation. Thus, I can reverse this and use it to assume that if I insult you, one of our parties will be hurt, and possibly I too. Both ways, the civility and stability of our relationship breaks down. This is a version of The Golden Rule – treat others as one would like others to treat oneself – with the addition that one can assume that he is likely to be treated as he treats others. If I commit murder (and we are now talking about criminal murder), there are the possibilities of revenge-murder (and it can be argued that capital punishment is the more modern form of this, although revenge-murder still may be done extra-judicially), or exile (the modern form being life imprisonment). In both cases, the role of a stable society – which is to protect all involved – has broken down with both parties damaged. By being a part of this stable society, one has a duty to uphold its premise. By making the decision of committing criminal murder, the murderer has undermined the premise of his society and failed in his duty. Therefore, society as a whole can condemn his actions. In addition, this duty towards maintaining a stable society is what deters one from not only committing murder, but also the other actions that we as a whole consider – quite simply and bluntly – bad.

The question of why we have a duty towards society is a separate one, and is more focused in political philosophy. This, on the other hand, is a question of descriptive ethics, thus it is sufficient to outline what is the deterrent to murder, and that I did.

- Basically an oversimplified and slightly botched version of Kant's ethics. Kant probably gave the best answer to this question with his catergorical imperative, in my opinion.