Monsters starts with a familiar childhood scene: a kid checking his room and under his bed for the monster everyone says is there. He looks everywhere and, luckily, finds nothing. The tone is light at first, almost gentle, like a bedtime story.
Then the poem jumps forward. The kid grows up and no longer checks his room or his bed, because he already knows where the monster is. The last line gives the twist: the monsters are humans.
The speaker sounds quietly disillusioned. Growing up hasn't made the world safer, it has only moved the fear somewhere else. The imaginary threat that could be searched for and ruled out has been replaced by a real one that can't be hidden under a bed. The short, plain lines help here, since they sound calm and matter-of-fact, which makes the final statement feel bleak and a little chilling. The mood moves from innocence to unease, ending on a hard truth.