That doesn't logically follow. An atheist can believe that they're objective and exist outside people, society, or even deities. Whether there is a God or not, murder is still wrong. Rape is still wrong. Theft is still wrong. We don't need a deity to tell us that. Rules against murder have existed in all civilized societies since before the time of Christianity, when they had never been exposed to the idea of a Judeo-Christian God (Jews had not yet had their Diaspora, so most people in the world had never even heard of them, much less their god).
If Jesus came back to Earth today and told us that murder and rape were okay, that would not make them morally acceptable--at least to an atheist.
Based on....?
Whether there is a God or not, murder is still wrong. Rape is still wrong
Why? Animals routinely harass and kill each other (even within species) and it is not considered 'wrong' for them to do so. Disturbing to us, perhaps, but not very much to them.
Rules against murder have existed in all civilized societies since before the time of Christianity
So? So has God, who has created humans after his image (having many of his attributes) whether or not they believe in him.
If Jesus came back to Earth today and told us that murder and rape were okay, that would not make them morally acceptable--at least to an atheist.
But this is a moot conjecture because if 'jesus' came back and said that, it would by definition NOT be Jesus.