why does god let bad things happen?

if god was real, why would he create such an evil world. god created humans, so can’t he stop people from acting in evil ways? if god created the earth, why would he create such things as drugs, guns, etc? my understanding is that god created all, and god created humans, and therefore created humans brains. if some sinful thing or evil source was corrupting gods perfect vision, wouldn’t god be able to help or save them in some way?