If God is all good, and God created everything, then how does evil exist?
The most simple answer is that only God is perfectly good—is, indeed, Goodness itself; he is the only being who is identical with his own goodness. Lesser beings, who are not infinitely perfect, are not identified with their goodness, the goodness of their particular natures, and so can fall short of the perfection of goodness. This possibility is not one that even God can prevent. If he creates beings other than himself, then they are capable of falling short of their own end or perfection. It is not that God wills this evil but that it remains always a possibility to anyone who is not Goodness in person.