Atheism is not a system of values
Atheism doesn't tell you what you "ought" to do. It is simply a lack of belief. Naturalism on the other hand is an epistemological position, i.e. "I don't see evidence for the supernatural, which includes gods"
One can be an atheist and be a great humanitarian or a mass murderer. Atheism is not a system of values. My lack of belief does not inform my values in any way.
But then I am not merely an atheist any more than a Christian is merely a theist. Christians are theists who specifically believe in Christ. They do not for example worship the Aztec god Huitzilopochtli, and thus they do not rip people's hearts out on altars as part of their worship of their deity.
I am a naturalist, a person who does not see any evidence for a supernatural dimension and therefore does not believe in gods, and I am a humanist. Humanism is an ideology that is predicated on the notion that we should increase human well-being and diminish suffering to the greatest extent possible.