Bad things happen to the world because of free will. Think about it: people are given the choice to do whatever they want, even if it's against God's will right?
So how can the world be good when it's full of bad people who are willing to do things against God?
And if God somehow forces the world to be right (which means removing criminals, gangsters, terrorists, mafia...) then that will destroy the whole meaning of free will right?
Also, you may wonder: what about natural disasters like cancer? Well, this is harder to explain, but have you ever think about what the world will be like WITHOUT cancer? Think about the population explosion! We're already in a population crisis where there is too many people on Earth. Our personal space is decreasing, our economy is crashing down due to limited resources, and people are going hungry because our agricultural output isn't enough to support the boom of human growth! So perhaps diseases like cancer is not so bad after all, if you think about it. It allows people to die, and the cycle goes on. I know this may sound harsh for those who have lost their loved ones to diseases (myself included actually), but the truth is, we do need ways to keep our population in check. a world where everybody lives over 100 is the day Earth no longer can support the human population