I beg to differ. My question ("...is it only a process of becoming comfortable with yourself?") is in earnest. You see, as one maps out an ethical framework in a Godless world, one necessarily has to have criteria by which to decide what to include, and what to reject. Your primary criterion ("'good' is that which promotes greater actualization of that universe through it's highest known aspect: life, human life") can only be interpreted subjectively by the individual. Therefore, even the most honest and stringent self-disciplinarian could well reach a point in this process where he says "I don't know all the answers, but I am comfortable with the ethics that I have constructed for myself thus far, and can be content with living within them."
And there he rests, having individually taken his best shot at "...that which promotes greater actualization of that universe through it's highest known aspect: life, human life."
What are we to think of such an individual? Are we even qualified to hold an opinion of such an individual? Remember, this is Godless space, and so our judgments carry no more weight than our own chutzpah can bring to bear. He has completed a rigorous process of self-examination, honest inquiry, and discipline, and ends up with no better assurance of having moved toward the collective ideal of "good" than having become comfortable with himself.
Speaking of which, there's a point that I failed to bring up before when it was more appropriate, but cannot bear to let it pass unnoticed... earlier you said explicitly that human life is the highest known aspect of our universe. That strikes me as an awfully egocentric thing to say in a Godless cosmos. Why should one complex electrochemical process consider itself "higher" than another?
Even interpretations or extrapolations from scripture will be judged with a subjective bias, even if it's by a clerical authority. But that doesn't detract from the objectivity of the source or negate the ultimate validity of one interpretation over another.
I'm not sure of the significance of the source of the ethics of the individual that you described. No matter how he reached the end of the road in his spiritual development, atheistically or theistically, it's his actions rather than rationale that should be judged. And our propensity to judge them is going to vary widely among both atheists and theists groups. In the end, a society dominated by either will develop a legal and punitive system to manage the most offensive behaviors and social systems to effect others.
Regarding the reason to place human life above all else known, I think at its base is a recognition of the development of the universe. From some unknown start to a single element expanding to stars and galaxies, to molecules of greater complexity, to life and then to a being uniquely aware, capable and accomplished. I don't think it takes "egocentricity" to recognize that pattern.
I suppose one could look at that entire progression as an example of decay, as a drain on the universe from its previous nature at rest or balance. If so, then one searching for a pattern in nature would have to value death.
From what little I know of cosmology, the development of stars, planets and life is the dominant story, and we're at its apex. One can be for or against the continuation of that progression (for or against life) but there's nothing that I'm aware of that's of comparable significance (theology excluded) that would challenge this basic pattern for a value system.
This of course presumes that people consider it of any significance at all. There was no model like this to compare to the certainty of religious values 100 years ago, but religions will connect with people on a more personal and deeper level than what I described, perhaps more so than any way it could be described.