Well, one can believe there is a real purpose in this life, that there will be ultimate justice, that there is a creator God who is good and we have worth to Him, and who wants to have a relationship with us,
you can believe this is all a cosmic accident, that right and wrong are merely just preferences, and ultimately what you do in life, be it a murderer or a life saving doctor, doesn’t matter because we all die and that’s it, and eventually our sun will explode and destroy any trace of us and wipe out the cosmic accident from existence. None if it matters.
Utilitarianism is a morality acceptable to atheists. It provides for a calculation of the pain or pleasure, somewhat like that promoted by Epicurius us. It is good if it gives pleasure and pleasure that does not harm the body, or those for whom we feel affection. It is bad if it causes pain that is hurtful to the body. The end in view is survival at a level of moderated pleasure. It falls over the refusal of so many humans not to follow this prescription for the good life. Bertrand Russell could not tell us why Hitler’s treatment of the Jews or our firebombing of Dresden was evil, except each was opposed by social convention.