The fact that the monsters only hear could also explain their motives for killing humans.
Whatever planet the monsters are from, they did most of their hunting with hearing and haven’t needed eyes to survive.
In order for the planet to be more suitable for their inhabiting, they must eliminate sound, which makes humans an annoyance and problem for A Quiet Place’s monsters.
Unless, they just enjoy killing people, we can assume that their motivation for killing people might be more from them having a survivalist mentality.