Then he is not omnipotent.
Is he able, but not willing?
Then he is malevolent.
Is he both able and willing?
Then whence cometh evil?
Is he neither able nor willing?
Then why call him God?"
--Epicurus, Greek philosopher (341-270 BCE)
Suppose you are tasked with deciding between two people which of them is good and which is evil, as if this represented good versus evil in the world. How would you go about determining this? To start, you may create a list of good things and bad things, and see how each person measures up. If you were to make such a list, murder would likely be near the top. Indeed, it is safe to say, that most people would put murder at the very top of their list. This is a universal concept which has been found in virtually every single civilized community for as long as we have records. Therefore, if one of the two people you are examining were to commit murder, it would be very difficult, indeed, to say that the killer is the good one of the two. If both had committed murder, then we might be able to say that they are both evil. But let's suppose that you have to make a definitive decision that one is evil and one is good. Well, then you would need to consider other factors like the number of murders committed, methods of death, and the reasons behind them. If one person committed a great many more murders than the other, used torturous or painful methods, and did so for petty or jealous reasons, that person would likely be deemed the evil one of the two.
Now, let's say that these two people are god and Satan (who didn't see that coming?). Of the two, which one has killed millions of people (i.e. Noah's Ark and the First-Born's in Egypt) through magic and various plagues and natural disasters, all because they wouldn't do what he had asked? Which one killed a single family (i.e. Job 1:12,19) because the other gave him permission, just to prove a point?
Given the death counts indicated in the Bible, if you believe god and the devil exist, how do you know god is the good one?
Here is the Atheist Experience tallying up Bible deaths: