Amoral people make bad friends. Maybe you can convert them to morality via mutualism and forgiveness, and establish an alliance with humans, but not if their intrinsic nature is harmful to humanity (instinct to kill, needs human flesh, etc.). Maybe the dumb monsters are just misunderstood too. At the very least it’s more nuanced than most monster-of-the-week battle harem stories where the monsters from the sky have no rhyme or reason. Though it might just devolve into a typical battle shonen with preachy overtones. We’ll see.