[untamed]

I can't believe that this would be the same critter most of us call a "Bristle worm". If it is the thickness of a finger, I would bet that it is a Eunicid worm. As someone already mentioned, Eunicid's are very distinctive because they have 5 large tentacles surrounding their head. They also tend to look more like centipedes, while bristleworms look more like fuzzy caterpillers. (no relation either way) Eunicids are also really fast, which probably contributes to why you haven't been able to get a picture.

Also, the evidence you have on giving it a death sentence is completely circumstancial. Fish die and you don't find the bodies. If you do find a body, the creature that is currently eating it probably didn't kill it.

I've kept many Eunicid worms over the years and never seen one do anything except eat scraps and vegetation.

I voted for leave it be, until you have some real evidence that it has to go.