In other words, whatever works best is right. Society experiments with ethical behavior to determine which set of rules works best. There are potential dangers in this kind of ethical system. But, to be fair, just because someone has an absolute ethical system based upon the Bible is no guarantee that he will not also join forces for the killings. But the issue is the base and ramifications of that base.
Beliefs affect behavior.
That is why belief systems are so important and absolutes are so necessary. A boat adrift without an anchor soon crashes into the rocks. In contrast, the atheists' presuppositions must be evolutionary. Since evolution teaches that life is the product of purely natural and utilitarian properties of our world, survival of the fittest, natural selection, and equating humans to animals as a species are the ontological basis for our existence and living. With this the value of man is lowered.