Atheism is a lack of belief in any god; no more, no less. Morals can be driven by a desire to do good in the world. Goading people into doing good deeds by threat of eternal suffering is boorish.
Abstaining from murder allows survival by forming alliances. Also:
Friendship – Treating people with kindness and respect sets a positive example. Killing other people seriously hampers long-term friendships and other relationships.
Kindness is good – If a god told an atheist to sacrifice her child, she would tell that god to go rip itself with a solar-powered chain saw while standing a quarter-mile from the Sun's searing surface. Theists might kill the kid.
Death is final – Murder devalues human life; socially well-adjusted people value others due to empathetic responses, which are not the exclusive domain of god belief.
In stark contrast, organized religions have justified murder, torture, genocide, infanticide, and slavery. If anything, propensity towards violence increases in proportion to the fundamentalism of a person's religious faith.
