He has a plan for everything, but when sin entered the world, way back with Adam and Eve, then the whole world was cursed. That brought in pain, disease, suffering, natural disasters, violence etc. It is not His will that this all happens, but He gave us all the gift of free will and choice, and when we choose wrong, there is consequences. These consequences are far reaching. It seems Satan has free reigh of the earth, but it is actually limited.