The point of the book isn’t just “people are the real monsters”, but also that evil begets evil. The creature is only ever shown hatred and cruelty, so it begins acting out as something that should be hated. By the end of the book, it has murdered multiple people. The victim eventually becomes the victimizer, which brings the tragedy full circle.

Simply saying “The creature was a victim” lacks any kind of nuance regarding the rest of the book. Yes, the creature was a victim. Then it became a monster, because it had always been treated as if it was one.