If I asked you to give me a reason not to murder someone, could you do it without appealing to religion or the supernatural?
Again, purely naturalistic beliefs have consequences - if we are merely animals - I can not give a reason not to murder - it happens throughout the animal kingdom and is part of natural selection - lions kill the cubs of other males and then mate with the female to continue the gene pool - we do not hold animals morally responsible for their actions - why are we morally responsible? It seems morality for humans does not help evolution - we save the weak and allow them to reproduce. How does morality only help humanity and not any other animal if evolution is merely about reproduction?
So, to answer your question, without objective morality there is no basis to not murder. It happens in North Korea, happened in communist Russia, happens in China, and eugenics happened here in the US.