Good and bad exist, in their own right. There origin be morality. For everyone there are different sets of moral views. Now, considering an atheist, as he believes in nothing as per se, then why would he save a dying dog, like inside, there's some moral instinct that make him do so, even though he might not be believing in its Creator even.
Meanwhile, as science is to matter, reason is to logic, life is to morality, death is to creator. If atheist is to science, he hath to go with logic & morality, even though he can't ignore death part, ( but he does the creator part, i.e outa logic). Now, its an atheist, who makes his own set of good/bad, as per his evolutionary mind, ignoring the Creator part for nothing, and that may sound illogical to a rationale mind.
And, maybe thats why, one opts ascetic ideal, ignoring morality, etc stuff as is evident from the birth of tragedy'.
As they say, one of the two things happen, after endowment of suffering, either one switches to spiritual mode or to a state of tots denial, to seek refuge, as per psychology. And latter happens in most athiest cases, perhaps.
