It is sufficient for the atheist to know that human nature is fundamentally social and that our individual development and well-being is directly derived from our social connections.
It isn’t a matter of wanting to be social. Being social in the natural sense is not a choice that humans make, but a biological imperative. An adult can choose to be hermit, but they can’t choose to stop being a social animal. An infant and child absolutely require social interaction and a stable community for their bodies and brains to even develop, and even the adult hermit is an exception that suffers many difficulties as a result. Being social isn’t a pragmatic choice for a human, it is an objective reality of our very natures.
If it is truly wrong to push a granny down the stairs, that has nothing to do with practicality but rather it is to do with the activity itself in reference to a universal objective moral standard; a law about how a person ought to behave.
It can be permissible to push Granny down the stairs even if it is circumstantial, and this is true even in Catholic moral law. Pushing Granny down the stairs is not absolutely wrong in every instance, for example if she is charging at your infant with a knife and intending to stab it. My point here is simply that even with firm objective rules there is nuance in how they are applied in given circumstances. I think you are referring specifically to murder, however, so I will address that.
The believer can say that murder is objectively wrong because God said so, while the atheist can say that it is objectively wrong because it breaks down the social fabric that is essential for human existence; the former appeals to Divine Law, and the latter appeals to Natural Law, but both are objective standards even if it can be difficult to discern the best course of action in a given circumstance. The atheist may have murkier guidelines and fewer restrictions in certain matters, but the basis of morality remains objective.
The bottom line is that human nature is an objective reality in relation to the individual, and human nature comes with some essential characteristics that provide moral guidelines for behavior. Obviously if we are looking for morality outside of human nature entirely then we must look to God, but when speaking of the individual we can indeed make objective determinations to a great degree through understanding human nature.
Peace and God bless!