these guys are nocturnal animals, they are scavengers and often eat decaying plant material. The thing I've noticed is they like to sit on the tops of the milkweeds, the seek refuge on the very top leaves, so the best way to keep them off is to pick off the top leaves. Though, the only thing I've noticed 'em doing is eating my milkweed, other then that, they don't do much.
Earwigs are sometimes cannibles, so yes, they are sometimes carnivores. It all depends on what they want to eat. If you think they may threaten your monarchs, just clip of the top leaves of your plants. Unfortunaltely, the flowers won't grow, so you won't attract as many other butterflies.