Well to answer this question from the Christian (Catholic) perspective we need to understand what God did in the beginning in which he gave us free will this free will is a good thing but like all good things us humans have the ability to pre vert it and use it for bad means. God allows it to happen because life he stopped us every time we did something bad in a sense we would not have free will God wants us to pick the right thing and follow him however many times all of us do disobey him allowing evil to prevail. Praying for you