The obvious answer to this question is that murder is wrong. Everyone believes that murder is wrong (although not everyone agrees on how to properly define murder), and the wrongness of murder is so self-evident that only a fool would even ask why murder is wrong.
Of course, I’m not asking why murder is wrong, at least in a moral sense. Instead, I’m asking why murder is illegal. This distinction may not seem obvious or even necessary, but asking why something that is immoral is also illegal can yield an interesting insight into the nature of formal governance as seen in the legal system.
The necessary question to ask, I suppose, is how did murder become illegal in the first place? Specifically, did governments pressure their citizens to give up murder or did citizens pressure their governments?
The answer is neither obvious nor straightforward. Certainly, citizens have murdered one another over the most petty of things. Cain murdered Abel because he didn’t like having his sacrifice overshadowed. Thus, it would seem necessary for intervention from above, whether that be God or the state. How else would murder be prevented?
On the other hand, the most egregious murderer of all time has been the state. Kings have been able to kill not only enemies, but citizens as well. Banning murder would limit the state more than citizens, given that the state is more prone to mass murder. And history suggests that the state is in more need of restraint than individuals. So, was the original purpose of making murder illegal simply an attempt to place limits on the state? Again, it is difficult to say.
There is a sense in which the government can mold social mores. But there is also a sense in which the government cannot change anyone’s mind at all. Violent crime statistics support these assertions. Violent crime rates peaked in the 90s, when criminal sentencing was at its most lenient. As sentencing became tougher, crime rates dropped. Reintroducing leniency to the system would undoubtedly yield higher crime rates again. However, eliminating all laws against violent crimes would not necessarily mean that everyone becomes a criminal. Furthermore, sentencing violent criminals to death with no hope of appeal would not entirely eliminate crime.
Ultimately, the question is: does the government determine morality or does it reflect morality? The answer is yes. The government can both set morality and reflect it. I would opine, though, that the government does the latter far more than the former. I would also posit that governmental attempts at the former have a strong tendency to be a justification for immoral behavior.