Bristleworms are not murderers (haha it sounds funny saying that). Bristleworms are primarily cleanup crew members, they eat dead animals and eat detritus. More likely than not, the animal it was observed eating was either dead/dying/or in a weak enough state for the bristleworm to kill it with very little effort (this requires the animal to be in a very weak state).
Keep in mind, there are some species of bristleworms that will eat corals, commonly referred to as fire worms. Out of the vast majority of bristleworm-type creatures, fire worms make up a very small portion of it.
Just forget about him IMHO, Im surey oull see him again trying to get leftovers. In some cases, fish may also eat them, even corals
Best of luck to you all
