I haven't looked too far into what particular Satanic groups proclaim, but I think you could construct a fairly unsilly story out of, for example, the Manichaean heresy.
Also, the hard part is not shouting "Good" (or "God") but deciding which is which. If you look at the actual Bible (perhaps through the lens of someone seeing it as the story as written by the victors) it isn't difficult to make the Abrahamic God look monstrous. And really, why should something be right just because God said so?
Hypothetically, suppose angels came down from heaven tomorrow, in public, and proclaimed for all to see that actually the Aztecs were right and asked why did the human sacrifices stop? Building pyramids is the wrong response to that, because whatever it is such a God wants it has nothing to do with "goodness." Even if it were futile, and if there were an evil omnipotent deity, I would argue that there is a moral duty to disobey such a God. Even if that God also tossed in some really good ideas to conceal the hideousness, we would be obligated to ignore the evil parts.
If it wasn't clear, yes, I'm an atheist.