An Atheist is someone who rejects the concept of God in the religious sense of the word. There are Atheists who entertain the thought of powers greater than ourselves, they just do not attribute those powers with supernatural ability.
The point is that "Atheist" is not a belief system. An Atheist is a person who may be altruistic or may not, There is no fear or love of Deity to drive his actions.
One last point worth mentioning. I have never met an Atheist who believed in a Devil, but I have met many Christians who work wickedness.