The problem with claims like "murder, unless in self-defence, is wrong" is that "murder" already includes elements of wrongfulness in it and self-defence already includes elements of rightfulness in it.
How does murder include anything else than elements of wrongfulness? Murder is always wrong, only exception is when it is used in defense of life or property. I cannot think of any other situation in which murder would be right.
How does self-defense include any other elements than rightfulness? Self-defense is always right. I cannot think of any situation in which self-defense is wrong.
I don't get the point you are making that the claim "murder is wrong, unless it is used in defense of life or property" is problematic.
For example, I shoot someone because they were trying to take a car. Is that "murder, unless in self-defence"? Well, yes if it's his car but not if it's my car. So you can't even make sense of "murder, unless in self-defence" until you already have a full theory of rightful ownership of property.
You just made sense of it. Murder/killing is wrong if you shoot someone that is trying to take his own car. If he tries to take your car, you are allowed to defend your property.
Someone needs a job at my store or else they'll die of starvation. I don't hire them because I want them to die. Is that "murder, unless in self-defence"?
No. The prerequisite is problematic. How can you know for sure if he is going to die if he doesn't get the job at your store? If it is a fact, you can voluntarily choose to save him, but if you don't you haven't murdered him.
You are introducing another moral argument, which is if people have an obligation to help each other. This is preferable behavior, but only if it is voluntary.
Someone needs a kidney transplant or else they'll die and I have the only matching kidney. I decide not to donate it. Is that "murder, unless in self-defence"?
No. Your kidney is your property. If him going to die is a fact, you can voluntarily choose to donate your kidney and save his life. But if you don't you haven't murdered him.
This is the problem with the NAP. It seems simple and seductive. But you can't actually determine what is or isn't "aggression" until you already have both an absolute notion of property rights and a notion of a scope of moral authority.
So you're arguing that the NAP is not valid because we won't be able to define property rights? And because we don't have a scope of moral authority?
I agree there is much more philosophical enlightenment necessary to define what universally preferable behavior is. But I think the NAP is the best starting point. If you don't have an alternative and we are not even able to agree on the most basic principle of non-aggression, there is no further use in discussing it.