This is one theory why God couldn't exist....
One: Assuming God is all powerful (obviously he is, he is God, if he wasn't all powerful, he wouldn't be a God)
Two: Assuming God is all good.
So, assuming these are true...which all Christian's do believe are true.
For there to be evil on earth, God must either one, not have the power to get rid of the evil, which would mean that he is not all powerful, meaning he is not a God.
Or
He is not all good, and accepts the evil on earth, which would go against everything in the Bible, because according to the Bible, God is all good.