Researchers discover new ocean predator in the Atacama Trench

Predator sounds dramatic, but there really isn't much non-microscopic life in the oceans that isn't predatorial. Large herbivores are land-evolved.

Would you consider the blue whale to be a carnivore, then?

blue whales eat krill, almost exclusively. so... yeah, definitely carnivores. i don't think there are any herbivore whales.

Considering they eat crustaceans, predominantly, wouldn't you?

While there are no herbivores there, many animals at that depth are suspension feeders or deposit feeders, i.e. they filter the water or the mud for either alive micro-organisms or for organic substances that come from the decomposition of dead animals or algae.

At that depth an important food source is formed by the dead bodies or parts of bodies of various big or small animals, which fall from shallower water after their death.

Because at high depths there is no primary production in most places (with the exception of vents where free dihydrogen or dihydrogen sulfide may feed bacteria), most animals must eat the dead organic matter that falls from above.

Predators that eat alive animals from that depth must be much fewer than the animals which eat dead matter, otherwise they will die of hunger.

Not really. It seems to me term filter feeder exists outside the whole carnivore/herbivore dynamic, which seems like less of a dietary thing and more of a behavioral thing anyway (though of course many animals are simply unable to digest stuff like cellulose, so it is also dietary... but then again these things are messy)