I think the author is leaving it up to us to decide who and what is good or bad. And to decide what we are going to, or want to learn from this—which I think is pretty cool, there aren’t very many authors who do this.
I don’t think there is such a thing as “no lesson”. As I just explained, I believe there is always something to learn whether or not it is a definitive, spelled out lesson.
As for why the monster keeps showing up, I think it is a blood-thirsty tyrant that highly enjoys playing humiliating and repulsive psychological games with it’s prey, to entertain itself before its meal is through. No. In all reality, I can speculate, but I don’t believe it is there to mentally or physically hurt anyone.