A defense of atheism has two parts. The first is to criticize arguments for the existence of God. The second is to argue for the nonexistence of God. Let me distinguish two arguments for the nonexistence of God. The first goes like this:

1. The proposition that God exists and the proposition that evil exists are inconsistent.
2. Evil exists.
Therefore,
3. God does not exist.

Most theists accept 2 and reject 1. The second argument goes like this:

1. The proposition that God exists and the proposition that gratuitous evil exists are inconsistent.
2. Gratuitous evil exists.
Therefore,
3. God does not exist.

This time, theists accept 1 and reject 2. Both atheistic arguments are valid, but the theist claims that both are unsound.