@Shostakovich phil,
People who kill don't seem to care about the laws of the state or country to begin with. Most criminals don't care about the law, if they did, they wouldn't be committing crimes to start with.
There is an odd discrepancy with states that have the death penalty and murder rates. It seems that a majority of those states that still have capital punishment for murder have the most murders still. This could be because those states originally had high murder rates and installing capital punishment was found necessary, but that still does not answer why murder rates remain so high in those states with it.
If you are correct in saying that the threat of capital punishment reduces murder then you should see the opposite with the US statistics, but clearly it is not the case. But then why is it not working? I say because a person who is willing to take another life, is not considering either in that moment, or that day or when ever, the results of their actions. I would go as far as to make the claim that those who are willing to kill plan on never getting caught for the crime as well. If you plan on not getting caught then technically they wouldn't care about the punishment either.
Another example of this is sharia law. I watched a video of a man getting his hand and foot cut off for a stealing conviction. This to me is rather harsh and the punishment is something the guy can never recover from. Everyone who sees him for the rest of his life will immediately know what he did. But the real question is, even though he lived in a place where he probably knew what the results of his actions would potentially bring, why did he go through with the act of stealing? Was he not aware that he might have his hand and foot cut off for stealing? If he did know, it did nothing to prevent his actions.