How about "harm" as a legal concept? Do you honestly think that harming someone to the point of taking their life is a concept that is rendered invalid in the absense of God?
I don't think it takes as much moral hand-wringing to answer these questions as you seem to think. I think you simply like hearing yourself propound seemingly intelligent sophistry.
I think you are asking if it wrong to harm someone to the point of death if there is no God. I'm sure we could all think of some utilitarian reasons for not harming someone - they are more useful to us alive than dead, etc. But morally, ethically, the answer is "No." Without God there are no eternal consequences and no eternal standards, so if you can get away with it, it is entirely up to the Mind of Man, and on that there is no Governor. I agree with Dostoevsky, who I believe said that without God, all things are permissible.
And such it is. The Apostle Paul, quoting the Prophet Isaiah, said that without Christ, "let us eat and drink, for tomorrow we die."