That's what they do best; eat dead things before they rot and kill other things. In the past, they've been accused of killing then eating but we now know that they will not attack healthy animals, even slow moving or sessile ones. If they're munching, it's already dead or dying.
There are a couple predatory species, but they either have very specialized diets (gorgonians for the Caribbean bearded worm) or are VERY recognizable (bobbit worm). Rule of thumb, 99 times out of 100, and unidentified bristleworm is still a good one!