"Humanity"? Remember, both Hitler and Gandhi were humans. It's too general of a term to apply to en entire species. It's true, though, that there's no other way to define "good" and "bad" than through a sentient species of your choice (humanity being the most useful one to us).
You assume that there is a good guy. There are a fairly large number of plausible scenarios that could result in an impostor impersonating a real creator (who simply doesn't care or got replaced by an impostor). Or you could have an impostor taking the credit for creation when there wasn't ever any such thing. You wouldn't even need to be divine or supernatural to get away with it. In fact you could probably get away with it with modern technology if you had access to the kind of technologically primitive people around 2-3 thousand years ago.
Why would you need an "impostor" at all? There's no reason for a creator entity to be good. Indeed, there is no reason for it to be even capable of being interpreted in terms of human morality. It's likely mindless, by human standards, or so much beyond our comprehension that we wouldn't be able to tell. That being would be working on massive spacetime scales, on which we are an insignificant thermodynamic fluctuation. It would operate by mathematics so alien that even our brightest minds wouldn't truly be able to understand them, though they might be able to work with them anyway (sort of how I manage to get anything done on the mathematical analysis course

). We're talking about a being to which 4-dimensional tensor algebra would be what being able to count is for a human. To create a world like ours, with all its intricacies, would take a mind completely beyond what is possible to achieve via evolution.
Indeed, the way I see it, the notion of an omnipotent cosmic being in any way interested in humans is far fetched. If there's God, he's likely a local phenomenon (just like we are), and
not the creator of the universe. He could be a creator of life, but that's all (here's a thought: if God is a local phenomenon, perhaps every planet has one. Ours is just more competent than, say, Venusian one and managed to make life that actually lasted). Also, he'd likely still be beyond our comprehension, but much less so than any that would be a global (universal-level) phenomenon. Evolution would shape the human species to conform with God's behavior, but it would not give us any way to actually understand the reasons for it. "Good" would merely be what makes God not to harm us, while "bad" would be something that makes him do so. We'd take it as something unshakable laws of universe, but they could very well results from completely meaningless (but consistent) behavior patterns of God.
Imagine rabbits learning how to run from cars. Those that do it right, live, those that don't, die. Over time, only the ones that do remain, and they will always run from cars the same way, and cross the road when they're least likely to be ran over. Yet, they never grasp the concept of a "rush hour" or even why cars move down the road, or why they exist, for that matter. It's bad to cross the road in the morning or evening, it's good to do so late at night. Our grasp of "good" and "bad" might work the same way, and come from something similar. God does not even have to realize we exist.
Yeah, I've been reading up on Lovecraft. Why do you ask?
