I think a lot of it comes down to Anne’s personal struggles with religion being reflected in her works. She seemed to have an on-again-off-again relationship with The Church (she was Catholic while involved with a church) and Big G himself. The question of the existence of the divine was a major theme throughout that series.

And then she went and retconed it in Prince Lestat. Memnoch apparently wasn’t the devil, just a particularly potent spirit.