I don't mean to speak for edgar, but in regard to the question of whether he's considered how the world came to be, he preceded that question when he said "there is no evidence that the world is other than just "is". The world exists, period. No beginning, no end. Some people give these attributes to "god," but "god" is an unnecessary complication. One can pare the middle man and give these attributes directly to the world. In regard to the Big Bang as it relates to the world, it is merely one process or phenomenon so far discovered by scientists within a larger process or phenomenon, as yet undiscovered by scientists, that we call "the world." There is speculation about this larger process that may involve an endless process of Big Bangs and Big Crunches.
As to the observation that materialistic foundations for ethics have lead to rationalized terrible deeds, and their comparison with terrible deeds based on hypocritical religious rationalizations based on selective interpretations of religious doctrines in clear violation of other fundamental principles, the materialists to whom you refer: the communists, clearly violated the purpose of communism as envisioned by Marxism: the betterment of "the people." Marx probably would have been horrified to see how his theory was perverted by the regimes to which you refer.
I can't speak for all of the atheists on A2K, but it seems that morality is something organic, that is to say, something that is a part of nature. It is seen also in other animals. Humans get along about as well as the other social animals. Sure, there are times when we kill each other, and have even eaten each other and our own young (the Bible itself speaks to this), but for the most part we get along and cooperate with each other. That's how we (all of the social animals) have survived in the world so far.