The universe came from nothing. Nothing comes from nothing. The atheist has no explanation for how life arose from non-life or how the personal arose from the impersonal. The atheist has no non-arbitrary way of declaring human dignity or value. Hence, the only difference between man and the animals is quantitative.
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.