Well, let’s give them the benefit of the doubt and say by altruism they mean “the spirit of kindness”, whatever essential part of their nature that causes them to want to be kind. It’s a subtle difference, but there is still a difference. Where does that spirit come from?
Kindness. It's still a circular argument which says the desire to do kind acts come from a spirit to be kind. Both start from man in this argument so both much start from the place. It begs the question. And apart from God what makes "kindness" special or any "better" than self seeking ruthlessness? In nature the big fish eat the little fish, what makes that "wrong" for men apart from God? Expecially in a Darwinist construct where survival is the only virtue?
Why is kindness desirable? If there is no God, why should we prefer to favor "kindness"? What is wrong with vicious brutality? Why should we not favor that which makes our own lives easier?
We've seen Atheist based standards of behavior. It was called "Eugenics" and it postulates that inferior people should be eliminated for the benefit of people possessing superior traits.
Apart from arguments based on spirituality, I see no rebuttal for the Atheist notion that the inferior needs to be destroyed.
They will not like the world that they think they want to create.