A good place to begin in finding out what kind of ethic one is already doing is the question: why did you become an atheist or agnostic? There are a multitude of possible answers with each the seed of a different way of being in the world.
Another type of atheist gives up his belief in God because he wants to indulge his carnal and venal desires. He probably still believes in God but professes disbelief instead. This type is most often encountered in Christian fantasies of the atheist in revolt against God.
A fourth type rejects God and Christianity in particular because they are deeply offensive. He no longer believes that Christianity is anything but a rationalization and justification of unfairness and inequality. For this type of atheist Christianity isn’t Christian enough. This one finds Christian values to be acceptable but finds fault with the administrative machinery that an organized religion requires.
A fifth type rejects God and Christianity because he finds Christian values and Christianity to be rationalizations for failure in this life. A good example of this type of atheist is Friedrich Nietzsche.
A sixth type rejects Christianity and God because they are an affront to his sense of autonomy. This atheist wants to manage his own affairs for himself. Christianity inevitably relies on (typically) men to speak for God. In claiming to speak for God these men speak only for themselves and their followers.
A seventh type can be imagined as a poor soul who envies the religious for the peace that comes from their faith. The self-loathing of this type comes from internalizing religious values but without a concomitant questioning and interrogation of those values.
An eighth type simply hates dogmatic attitudes. Being able to change your mind when presented with new evidence and arguments is the essence of honesty, integrity, and inquiry. For this one “faith” is just another conviction to be interrogated.
This list is by no means exhaustive. I came up with these eight types in few minutes. There are most certainly other types that have been overlooked. This list shows that the moniker “atheist” covers a great deal and means little more than a individual who rejects belief in God and (probably) the supernatural. Contrary to PZ Meyers ( Why are you an atheist?, Febuary 2, 2011, and Atheism and the Real search for Meaning -- January 8, 2013)“atheist” as the lowest common denominator means dictionary atheist. Our terminology seems to need revision.
These types are rarely embodied in a pure form in one individual. More commonly individual atheists are mixtures of more than one type, but in a given individual one type will predominate and be the answer most commonly given to the question, “why are you an atheist?”