Humans define morality, not some hidden deity.
Yes, I am convinced atheists believe in moral relativism, even when they are reticent to admit it.
I never said anything about a diety. I am agnostic about the origin of human conscience. I don't think Darwinian evolutionary biology explains it.
You can't say murder, theft, rape are objectively and absolutely wrong unless you subscribe to an unchanging moral law which isn't subject to human opinion and popular consensus.
Taking a scientific approach to life, murder and rape may sometimes be necessary. Male lions will kill the cubs of rivals. Male deer will overpower and force female deer to copulate. We don't call it murder or rape because we intuitively know human conscience is in some respect separate from the scientific world.
Spartans abandoned weak babies, and Vikings practiced female infanticide because the community would only benefit from the strong, and by having more males.
The Nazis defined moral value as eliminating inferior races and disabled people, because they were a threat to a healthy and robust Aryan population.
I am pretty sure human conscience in mentally healthy adults either perceives absolute right and wrong, or can ultimately be persuaded of it. When people are slowly persuaded through moral argument that human sacrifice, infanticide, slavery are wrong, those societies never go back to it at the institutional scale. That's not a coincidence.