Human value is a sliding scale. From the atheist perspective there can be no moral absolutes. He can never say why I should act one way rather than another. There can only be personal preference. Morality is a matter of taste, the will of the majority or based on reason.
Atheists can never say it is wrong to kill.
He has no final authority to appeal to other than convention. If there is no final judgement or a final judge in the next life, then there can be no true justice on earth. If there is no god there can be no explanation for the laws of logic. Therefore, the atheist can never prove his position. To use the laws of logic is inconsistent and self-defeating.