I agree that the ethics behind murder have no objective rationality, that is, the only rational arguments against murder involve it's consequences and not the act itself.

As you say, murder can only be argued against with reference to outside factors, such as empathy ect. In and of itself, murder cannot be rationally argued to be wrong (though it certainly can be argued emotionally. Don't get me wrong, I do not advocate murder).

The only sound logical arguments concern the effect of the murder on the perpetrator. Feelings of guilt can be overcome, same with the effect on those close to the victim. Therefore, the only considerations left are that of the wider implications of murder on society (as already mentioned), and the potential of retribution. The only rational way you could convince a sociopath not to kill is to remind them of the punishment they would receive if caught. It is highly selfish, but the only rational argument against murder is that it may lead to punishment. And even then, such an argument only works on those who care about themselves.

In the end, murder can only be logically argued to be wrong because you may be caught and punished. It all comes down to self interest.

__________________
It's the difference between suicide and slow capitulation... - Jim Morrison