God does not want evil things to happen. If he does, He would have created evil at the beginning but he didn't create evil. The Bible says that He saw everything that He had made, and behold they were all good. It even concluded that all the God created was very good. How could God give something evil to the world when the Bible says that every good and perfect gift come from Him? God does not want evil things to happen.
Let us remember that the Bible says that the imaginations of man’s heart are evil continually. That means that it is humans that fill the earth with all the evil things that we are seeing. The wickedness in the hearts of men is what is pouring out in the world. The reflection of the world is out of the fullness of what is in the heart of men.
The beginning of evil in the world came from the disobedience of man. When man turned away from the commandments of God and follow the devil, the devil planted evil in their hearts. That is why they have fill the whole earth with evil.