Musing on atheistic ethics
Some unbelievers insist that our "moral" sense probably all began when genes that favoured community were exchanged in lower animals at some stage in our evolution (rather than genes that were more immediately linked to survival at a sheerly physical level—the "pack" sense can certainly be considered as survival friendly). From that point on "communities" and "families" developed and the "community" genetic pool was enriched as well as enlarged. If people like bio-ethicist, E. O Wilson, are right and this is how mutual care and the sense that we "ought" to treat one another well came to humans, at least we have a clear picture. It's just another one of those products of purposeless and mindless evolution. There's nothing "transcendent" about it—we're genetically programmed to be loyal and caring. Of course, Wilson insists that the other side of the coin is also true—we're genetically programmed to hate and oppose each other. If you're not of my community or tribe or pack there is that divide between us and if it comes to a choice the "outsiders" go down. [Some that believe that, think we should nevertheless control our "selfish genes". But why "should" we? Where does "should" enter?]
However the fine details are worked out and the balance between "nature" and "nurture" (genes and environment) is settled, when we're done "morality" and "immorality" is physically based and came about by chance. We use words like "good" and "bad" and "evil" and "immoral" and "loyal" and "treacherous" and "greedy" and "benevolent" out of habit and convenience. We saw these as "virtues" in an age when we thought that God had something to do with them but now we know better. There's nothing "good" about goodness or "bad" about badness. These feelings and habits simply are what they are—the end product of evolution from inorganic compounds to "the bags of chemicals" that we are.
For our own survival we encouraged the traits that made for community and penalised the traits that threatened our community existence. But in favouring the "good" traits we weren't being good—it was another evolutionary urge fuelled by stimuli including environment, chance, pleasure, physical necessity and so forth.
As we evolved upward some branches of the evolutionary tree became more "intelligent" and so more powerful and shrewd but the "intelligence" grew out of the same soil as everything else; inner structural drives meeting the environment all of which rose up out of reality that is totally, utterly and altogether "material". The human branch of evolution outstripped the rest of the bio-world and took control of it. As a matter of simple survival it structured "societies" and adopted societal rules that outlawed anarchy and as a matter of course those societies coerced the citizens into acceptable behavioural patterns.
How the thought of God came into the picture is another story but with his entrance and the entrance of religion these traits that were created by chance chemical/genetic/environmental drives were given a super-natural character. Since there was no God and since everything from stem cells to black holes to killer whales, human DNA and supernovas—since these are all the product of forces and matter operated on by chance and the "laws" of physics "morality" and "goodness", "immorality" and "badness" have no real referent other than what humans choose them to mean. (Of course, "choice" is also a chemical based thing so there's really no choice.) But depending on who has the power to coerce, these words will change their meaning. In addition, the weaker element can only be coerced into submitting to the societal laws but as soon as they have the power everything may change and no one can rationally call their choices "bad" or "immoral" since these words have only a relative meaning. Different strokes for different folks.
Of course, if the "ought" realm is nothing more than divergent genes and should therefore be viewed as nothing but chance development in light of existent physical "laws" that came into existence at the Big Bang then ethics/morality mean nothing but what is. Your ethical stance (providing you aren't utterly amoral) is interesting, it has a physical basis and you may be able to coerce me into behaving in line with your ethical views but there can be no rational talk of moral high ground. Should I choose not to accept your ethical standards you might well coerce me into conformity but you mustn't pretend that you are "morally right" and I am "immoral". You mustn't pretend that there is moral right and wrong. You can only claim that how I choose to behave is different from how you do but both our "choices" are the product of sheerly genetic/chemical/environmental pressures.
If a Nazi has formerly behaved with "propriety" but now chooses to torture and murder he has simply thrown off one form of behaviour and adopted another. Murder and torture, then, is not a "moral" or "ethical" issue—these are just words that describe empirical activities that we don't like or are afraid of or recoil from. Call what Nazis did anything you like but you can't give it a transcendent nature, you can't feel morally superior. Mengele, Pol Pot, Stalin and the like did what they wanted for whatever reasons but for an atheist to condemn such behaviour as "immoral" or "evil" or "bad" is one bag of chemicals reacting to other bags of chemicals.