Me from another thread:
[...] objective morality, even as a theistic construct, only exists insofar as it is a reflection of the biological and scientific realities of the human race and its existence. Theism merely mirrors several ideas of morality that are more universal than others (shalt not kill and steal and a couple of other ideas) that came about due to humanity's engagement with the realities of its existence.
Those concepts of morality that have a deeper root in such biological and scientific realities are the ones that will survive for longer periods of time and are the ones that people most often label 'objective', whereas those that are established by discourse more separate to this are the ones that are more fluid and have broken down over time.
That is to say that killing people is not considered wrong because there is something inherently evil about it, but because, biologically, there is something inherently socially self-destructive about it. Without the development of some kind of discourse or ideology that condemned indiscriminate killing of fellow humans, complex society would never have emerged as it did. It is around this reality that discourses of evil and good are constructed by humanity.