I'm confused about the Universally-Preferable Behavior concept (as described by S. Molyneux). Help!

I've listened to a few of Stef's podcasts on freedomainradio.com regarding the universality of moral principles.

Essentially, he says that murder is immoral for Mr. X so it must be immoral for Mr. Y; that doesn't change if Mr. Y wears a green costume and is deemed a soldier.

From what I understand, the key here is that the same physical action cannot be evaluated differently morality-wise for two people.

But my questions is:

Can't I simply bypass this concept by re-defining murder so as to exclude the case of a killing mandated by a democratic government? This would conceivably make murder wrong for all yet morally allow a soldier to kill.

I would think that the solution to this issue is to define murder to involve the initiation of force, in which case Molyneux's statement would make complete sense to me. Doing this would require a more basic agreement that equates a soldier to a "regular person." So then does the social contract argument actually precede the UPB argument?

Thanks everyone!