Epicurus — kicking God's arse since 300 B.C. !
The problem of evil is one of the best and most debated philosophical arguments against the existence of god. Actually in my opinion, it is the best argument ever! A modern version of this argumet is follows something like this: Let, God be defined as an omnipotent, omniscient, omnibenevolent, ontologically basic, non-contingent, disembodied mental entity.
1. If god is Omnibenevolent, he would want to prevent evil.
2. If god is omniscient he would know every possible way in which evil may occur.
3. If god is omnipotent then he would act in every possible way in order to prevent evil from manifesting in every possible way.
4. God is non-contingent which means god is either necessary (necessarily true) or impossible (necessarily false).
5. Assuming god as a necessarily true existent being, god exist necessarily in all possible worlds.
6. In a world E, (subjective, objective or gratuitous) Evil exist. In that world E, also god exist.
7. God is omnipotent, omniscient and omnibenevolent which leads to a contradiction with manifestation of Evil in the world E.
8. Therefore according to proof by contradiction, such a god is impossible to exist.
9. Therefore, God does not exit.
Here I just presented a general version of the argument. As I read more about this topic, sophisticated arguments and their implications will follow through.