Wow. If mon mon mon Monsters had been made in the US, it would most likely be considered a modern "classic" like The Witch and It Follows are. Although personally I think it's much smarter and deeper movie than either of those films.
Reading some of the reviews here, most people seemed to gravitate to a "who's the real monster?" message, but that didn't come across as a question to me. It's bleak as hell, but the bullies are vile from the beginning of the film. The monster girl never does anything against them. It turns out they abuse just because they can. Because it's all they know. Because it's how they cope. Because they've been abused.
Violence has no end. Violence is born out of violence.