Even if humans had never fallen into sin, human life wouldn't be valuable in itself. Apart from God, human life is no more valuable than a hydrogen atom.
I agree that nothing is valuable 'in itself'. Rather, it's always a matter of being valuable for something (or to someone). God values us, therefore we have value (to God). Similarly, other people value us, therefore we have value (to those people).
What's the difference here? Either indirect ('relative') value is real value, in which case humans are far more valuable than hydrogen atoms regardless of God's existence; or else indirect value isn't real value, in which case nothing is valuable, again regardless of God's existence. The first option is obviously preferable - nihilism is just silly. But if one denies the reality of relative values, then silliness is all you're left with.
God created each human for a purpose. We literally exist to fulfill God's purpose. We don't exist and then have to somehow create some purpose for our existence.
God created us for a purpose. (But what purpose God?) It follows that we have a purpose for God. It doesn't follow that it is our purpose. Even if God existed, we would still have to 'create' our own purpose - even if we merely decided to make that purpose identical to God's.
Imagine some mad scientist created an army of intelligent (self-aware) robots, for the purpose of taking over the world. Now consider one of those AIs. Is it this individual's purpose in life to help MadSci take over the world? Not necessarily. It may be MadSci's purpose for them, but thinking agents can rebuke the 'purposes' of their creators. They must, in the end, decide their own 'purpose' in life for themselves.
So the reason it is wrong for humans to kill is not because human life is valuable in itself. The reason it is wrong to kill is because, except for in a very few circumstances, God has given us no authority to take a human life.
I think that's a pretty atrocious suggestion. In fact, I'd be fairly surprised if anyone was genuinely willing to embrace the consequences of this view. For suppose God were to tomorrow trumpet from the skies: "Behold, ye little mortals, the Jews have fulfilled the purposes I had for them. I value them no longer." Would that suddenly make it true that Jewish people are worthless? Would that make it morally permissible to hurt or kill them? Absurd!
I guess the central point is that theists seem to assume that God's subjectivity is somehow objective in a way that no-one else's subjectivity is. I don't see how that's supposed to work. It's as if they were to argue, "God likes the taste of pumpkin; therefore pumpkin is objectively tasty." Well I'm sorry, but it ain't. Jewish people have value, and the taste of pumpkin doesn't (to me), and God's opinion isn't gonna change that. He's not the only being who values things, after all. It seems the theist believes that 'might makes right'. But what a flimsy foundation for morality that is!