I never said atheists have no personal morality. I absolutely believe man has free will.
My question was, and still is, WHY is murder wrong? If we are just a bunch of molecules thrown together and we have no ultimate purpose. Survival of the fittest. Why don't I, you, or your average person, just go out and off another person? Where does that moral compass come from?
If you are an atheist and you believe survival of the fittest, why doesn't that apply to humans? If your great ancestor was a rock, why can't you kill that evolved rock (human)?
An atheist (a person who does not believe in god) and a psychopath (a person who does not experience empathy) are not the same thing. The pre-frontal cortex of a normal human being, whether atheist or theist, is the region of the brain that generates a emotional response to the actions and expressions of other people. The capacity for empathy for others is sufficient in most people, regardless of religious belief, to reject murder.
If you want a more formal justification I'd suggest picking up nearly any ethics textbook that discusses systems of morality (Kant's categorical imperative, Mill's utilitarianism, ethical egoism, etc.). You'll find that with the exception of divine command theories, just about every model of morality rejects the idea that morals are determined by god and that each model is capable of justifying prohibitions on murder, even if they do it for different reasons.
Here's some very brief examples of how different moral systems can justify prohibitions on murder.
Kant:
An individual should behave in a way they want everyone to behave
Everyone committing murder is not desirable
An individual should not commit murder
Mills:
An individual should act in a way that maximizes happiness of everyone
Committing murder does not maximize happiness of everyone
An individual should not commit murder
Egoism:
An individual should act in their own self-interest
Not being murdered is in each individuals self-interest.
If the consequences of an action outweigh the benefits, the number of people choosing the action is reduced.
To reduce each individual's chances of being murdered, groups of individuals (aka society) should create consequences for murder that exceed the benefits.
An individual should not commit murder because the consequences outweigh the benefits
'Survival of the fittest' is a description not a command. Acknowledging that natural selection occurs does not create a requirement that natural selection must be allowed to occur without human interference. Acknowledging that bad eyesight would be a handicap to survival does not require us to refuse to use glasses to compensate for bad eyesight. Even if it were proven that being a murderer was somehow beneficial to individual 'fitness', it does not logically follow that these more fit individuals must be allowed to kill.
As a society, we say murder is wrong. Why?
All societies say murder is wrong for the simple reason that no society can exist without saying murder is wrong. A society is at its basic level a group of people living/working together. No rational person will voluntarily live or work with people that they think will kill them at any time. Self-preservation would cause individuals to either live completely alone or to split into smaller societies where murder within that smaller society would be prohibited.