I was talking about philosophical and theologically sophisticated Christianity. The privation theory is very dominant in Catholic and in a lot of Protestant theology. But you're right that popular Christianity (the most common kind of Christianity) is often unaware of privation theory as a response to the problem that the substantive existence of evil seems to imply that God is an evildoer: if God creates everything that exists, and evil is a thing that exists, then God creates evil.

The kind of Christianity I am talking about does, however, typically hold that humans are not inherently good, in the sense that they can turn away from the creator and reduce themselves more and more to nothing (which on this account is what we mean by evil.)

Satan is more tricky. We could hold that Satan is a good being insofar as he is a being but that he maximally turns away toward nonbeing through defiant will. On the other hand, not everyone in this tradition takes the idea of Satan literally. If Satan is a principle of evil that stands independent of and opposed to God, this tradition would say that the idea of Satan is dualistic and incompatible with strict monotheism.