The argument from evil
The usual answer to the question why is there evil from christians is humans are free entities, and therefore can freely choose to do evil things (this is in addressing human evil and not natural evil or suffering).
But I'm not sure either type of evil is justified (unless the believer contends that they are good). Let me explain:
Gods nature is one which is not chosen by him, is not changing, and is objective. Gods nature, then, is impossible to escape. For example, his nature is logical. You cannot make a circular square or smell the color blue. Gods nature is something he and all his creation is bounded by. How, then, can anything exist outside his nature, and how can anyone choose to do something outside his nature? If humans are able to choose to do something outside his nature, then isn't that something which must be explained? We were created by god and given no other attributes beyond his nature, so doing evil would then be akin to seeing colors outside of the spectrum, would it not?