One of the biggest questions we atheists usually have about religion, and Christianity specifically is "If your god is a benevolent god, why does he let us commit terrible acts, acts that many Christians would call 'evil'?"
Their answer is usually that "God gave us free will" and that it is our fault for choosing to sin (such as Adam and Eve eating the apple). I think this is a cop out answer, as most things like free will and justice are human constructs, concepts that work in our environment, but that could seem alien to beings who's minds work differently to ours. Surely if this god was all powerful, he could create a world in which both free will and an abscence of sin can be obtained.
But my main point takes the form of a series of questions, which when answered should cast doubt into any rational christian's mind.
1. Is 'God' infallibly good? (The answer will undoubtably be yes)
2. Does 'God' have free will? (Yes)
3. Could 'God' ever do anything considered bad, or evil? (If the answer isn't no, I'd be surprised, but if they say he is doing it for the right reasons etc, the point still stands)
4. Didn't 'God' makes us in his own image? (Most will say yes, however since this statement is in the old testament, some Christians may refute it)
5. Then why isn't our free will like 'God's' free will? Either 'God' does not have free will, and we do, or we have a different version of free will to 'God'. You could say that 'God' chooses to do good all the time, but if he didn't want evil to exist, surely he'd make a universe filled with infallible, all powerful deitys like himself. In fact, I find it evil that a god that can do whatever he likes decides to make a species who's only purpose seems to be to bow down to him.
So to conclude, you cannot just cite 'free will' in humans to be the cause of all our problems, because if we were really like 'God' we'd do good all the time.