Wait a minute.
"Agree"? If Atheism is true, then nobody needs to "agree" to be abused. It can be done whether they like it or not.
If "survival of the fittest" got us here, why would we abandon that? Those that have power can freely tyrannize those who do not have it. They will never be called to account for their exploitation of others. So long as they can get away with it, why shouldn't they abuse other human beings? You're surely not introducing the idea of karmic cycles or some idea of ultimate justice into one without God...
In a world devoid of God, one fact is very obvious: might makes "right." Or rather, nothing makes "right" or "wrong." Those values don't exist! And "agreement"? Who is there to say that even counts?
Actually, this "grounds" nothing.
Some extreme perverts may agree to be victims but I believe 99.9% will not agree to be victims of the above atrocities.
- various atrocities and horrors, such as genocides, terrorist attacks, mass murders, and tortures and killing sprees by psychopathic serial killers.
Thus it is not difficult to get a consensus from the 99.9% to agree to these evil acts as absolutely 'No' 'Noes' in the secular sense. Therefore we can ground this as a secular absolute and to avoid moving goalposts. Such atrocities are obviously of high net-negativity to humanity.
Who says we owe "humanity" anything? Who says "consensus" matters? We have the power...that is all we need. And in fact, if Nietzsche or Darwin is right, we do a great service to the overall condition of humanity when we let the weak die at the back of the herd, and we, the strong, survive, dominate and procreate.
"Will to power" is what matters, said Nietzsche. Let's get rid of this nonsense, and get "beyond good and evil." That's the real "transvaluation of values" (all Nietzsche's terms, not mine).
Or take Rand. According to her, it is the great men who advance society. The sheep get us nowhere. The great men, the heroes, must practice "the virtue of selfishness," and use their own inner compasses to set their course. "Agreement"? That would just hold them back, and society as a whole would suffer because of it.
"Agreement" indeed! "Consensus"? Phht. "Humanity"? They don't know what's good for them, and don't have the courage to be "bad." Let the sheep "baa": the wolves have the advantage of them. Let the wolves thrive!
Rand and Nietzsche would agree to this part. They would say, though, that these "net-positive" acts aren't "trade-offs" at all. The survival and dominance of the übermensch (Nietzsche) or the John Galts (Rand) of the world, or of "the fittest" (Darwin) would be a positive for the race, so there's no reason to say any bit of it is "negative." Who cares if the weak suffer? They're the weak. If they deserve to live, they will; if they don't, it just proves they didn't deserve to live. Pity is a vice, not a virtue.As for low to medium evil acts, there are possible trade offs for the individual and humanity if say the 'evil' acts are committed for the greater good, self-defense and the likes. In this case, these acts themselves are evil in general as defined but the overall consequence is net-positive, thus not a resultant evil act for the individual.
What I'm saying is this: there are powerful ways of telling the story that deny the very fundamentals you are trying to use in order to affirm a secular ethic. So your project is troubled -- but not by religionists. It's troubled by the avowed Atheists (meaning Nietzsche, Rand, Darwin, et al), who will accuse you, under Atheist assumptions, of a lack of nerve, and of a less-than-moral squeamishness about doing what needs to be done in order for the human race to "go forward."
What will you say to them?