In Christian theology, a classical definition of evil is found in the work of Augustine of Hippo. He states that the evil is not a thing itself, and therefore is not brought into being by God. Instead, evil is the privation of right order. Or, to put it more simply, evil is the state of affairs when things are the way they ought not to be. So, if a mugger mugs you and steals your money, that was evil, because humans ought not to do that. And if a tsunami leaves thousands of people homeless, that’s evil, because the world ought not to be like that. (Let’s bracket why God might allow natural evil, such as the latter example, for another post).

The point is that when you talk about evil and suffering, it pre-supposes that the world is not the way it ought to be. But that means that the world ought to be some way. If the world “ought to be” any way other than it is, then that pre-supposes a designer, who had a purpose for the world, i.e. – a way the world ought to be.

So, atheists cannot use the apparently gratuitous evil in the world as a disproof that there is a God until they define what they mean by evil, and explain how this objective standard of good and evil came to exist.

So what is evil on atheism? An answer that is NOT open to atheists is the solution above, namely, that evil is a departure from the way things ought to be. Because the universe is an accident on atheism – it is purposeless – there is no way the universe ought to be. We are accidents on atheism. There is no way we ought to be.

So evil must mean one of two things on atheism:

1. Evil means something that the atheist finds personally distasteful. It is a subjective preference that each person decides for themselves. Just as some people don’t like broccoli – some people don’t like murder or tsunamis. It’s up to each person. But that cannot be used as an argument against God, because who says that God’s moral purposes ought to be connected to the personal moral preferences of atheists? It won’t work.
2. Evil is what society says is counter to the social conventions of a particular time and place. If we decide that murder is against our society’s conventions today, then for that time and place, murder is “evil”. But then, not signaling when you turn right at a stop sign is also “evil”. It’s all just made-up conventions. And again, it is difficult to see why God should be bound by a society’s conception of good and evil, they are just conventions of accidental people, on an accidental planet, in an accidental universe. (Again, we will bracket the problem of deciding what a society is for this discussion).

Neither of those options is going to allow an atheist to claim that God is evil. Because their basis for saying so is either going to be their personal preferences or the arbitrary conventions of the culture they happen to live in in arbitrary time and place.

So, it seems to me that pressing the problem of evil is inconsistent on atheism. There is no moral standard that an atheist can use to hold God accountable, in an accidental universe. You have to pre-suppose an objective moral standard, and a designer of the universe who makes that standard and makes it applicable, before you can proceed to hold God accountable to that standard. But then, you have already assumed God in order to argue against him.

To learn more about the difficulties that atheists have in making sense of morality, I really recommend this lecture (MP3) on the problems of evil and suffering by Doug Geivett (hosted by Apologetics 315), and this short 4-page paper on the problem of evil as well.