Imperfection is a major theme throughout all of the books. The ones that seem perfect on the surface like Lockhart and …Umbridge… are nearly always the worst. Even Quirrell seemed friendly.

I personally view Lupin as an allegory for alcoholism, being kind by day and cruel at night. Needing a special serum to maintain himself, and the full moon being a relapse.

As a whole, I think the story is about learning you cannot rely on the heros or adults above you. You need to grow yourself.