You can justify it with an economic argument. I’ll explain this below.
It could be, but it doesn’t have to be. Look at anything you own. Does it have value? Why? It has a value because you assigned it a value. Now, the way you made that assignment is complex and going to be based in part on how the rest of us value it, but ultimately you had to assign it some value, right?
I’m going to make this argument with a theft example, because I think it’s easier to illustrate. But you can apply this argument to murder and slavery if you want to:
Let’s say Person A has a painting that he values at $10. Person B wants to buy it, and B values it at $30. So B offers A $20 for it and A agrees. Now, B is ahead $10–he got a painting valued at $30 for $20. And A is ahead $10. He sold a painting for $20 he valued at $10. This transaction is economically efficient* and the total wealth of the system has increased $20.
Now, instead, lets say that Person B smashes a window, breaks into A’s house and steals a painting. B is ahead now $20 (remember, he valued the painting at $20). But A has lost $10 (the painting) and another $15 (for the broken window). So, B is $20 ahead, but A is $25 behind, and the total wealth of the system has decreased by $5. This is economically inefficient.
So, if you don’t want the total wealth of society to decrease, you ban economically inefficient transactions such as theft.
I can do this argument for murder and slavery if you want, but it’s pretty much going to be the same.
*I realize I’m using a specific definition of economic efficiency, and we can spend a lot of time discussing whether this is the appropriate definition to use. But, this isn’t an economics thread, and I just wanted to show that one can make economic efficiency arguments in favor of laws that prohibit murder, or theft, or slavery.