I would venture to say that the Golden Rule, which cuts across religions and cultures, might be a good start. And the Wiccan Rede: Do what thou wilt, an it harm none.
If they chose to obey the golden rule that would simply be their personal preference. Or they can chose Power, per Nietzche, and that would also be a personal preference. Neither one is better than the other, its simply a choice, and in the end who cares what the choice is as we all end up in oblivion anyway.
In any case, Athiests are responsible for the worst genocides of all time, Stalin, Mao, Pol Pot, Castro etc. And that makes sense given that, in an Athiest world, anything goes to get the job done, and getting the job done is considered “good” and “moral.”