That's usually true, which is kind of funny, because in their worldview there exists no reason to prohibit murder.
There is absolutely a reason to prohibit murder, athiest or believer. It is uiversal, disputed by none.
Given a murder, would you prefer to be a) the victim, b) the murderer, c) neither, or d) makes no difference/don't know. The fact is no one selects option a). It is not my opinion vs. somebody else's. Nobody wants to be murdered. That universality is the reason to prohibit murder.
There is a similar universality regarding slavery. Nobody would choose to be the slave.
So what? It's all matter in motion, right?
If you found yourself enslaved, as an atheist to which transcendent moral principle would you appeal to deem your condition unjust? In an atheistic universe, no one is equal. There are only physical characteristics. Christians hold that all men are equal before G-d, and because the civil law is an attempt to approximate the Law of G-d, all men should be equal before the civil law. If all men are equal before G-d, then it is an injustice and sinful for man to treat other men in an unequal manner.
If we ae nothing but hairless apes, then the idea of the strong dominating the weak is perfectly natural. Look at nature.
Not true, therefore your conclusion That universality is the reason to prohibit murder.
isn't valid. There is a similar universality regarding slavery. Nobody would choose to be the slave.
Again, not true.
Given a murder, would you prefer to be a) the victim, b) the murderer, c) neither, or d) makes no difference/don't know. The fact is no one selects option a). It is not my opinion vs. somebody else's. Nobody wants to be murdered. That universality is the reason to prohibit murder.
This is not a personal attack against you, but this argument is ridiculous. While I agree that murder is morally wrong, your attempt at a "universal argument" at least in the way you've shared it is untenable.
Structure your argument in a formal way:
Premise 1: It is universal that no human wishes to be murdered
Premise 2: Acting against a human's wishes is wrong
Conclusion: Therefore, murdering a human being is wrong
While we could find a nutcase who would theoretically "disprove" Premise 1, the weakness of this argument comes when reviewing Premise 2.
I could just as well say that no human wishes to be imprisoned, but of course society must imprison some. Or we could get more trivial and say that no human really wishes to pay his electic bill...
In summary, your argument as presented fails.