Now for Augustine, all goodness either is God or comes from God. God is of course the pinnacle and paradigm of goodness. Whatever is good in creation is good because it reflects in some way the goodness of God. Sin comes in when you try to have the goodness of a creature apart from the goodness of God. Notice, sin is not the desire for something evil.
Evil is nothingness.
You can't desire nothingness, you can only desire something. Augustine even argues that the suicide doesn't will nothingness or non-existence, although it might seem that he does. Rather, the suicide really desires peace. And peace exhibits the constancy that is the most conspicuous characteristic of Being. In other words, the suicide may think he wants non-existence, but in fact he wants existence of a higher kind, existence more like God's.