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.
Tangent: makes me wonder if fossil fuel depots that accumulated under the oceans might have a similar "they don't make them anyone as they used to" as the lignin/fungus imbalance of the carboniferous (I know that there are studies questioning that link, or at least suggesting that this can't be the whole story): before those feeders evolved, there would have been undisputed accumulation, and the first few evolutionary matches might have died out after burning through deposits in waves of overpopulation and extinction, before stochastics hit a stable balance. But, well, I'm just a speculation layman, experts will still have enough uncertainty for a wide range of speculation (see the wide range of uncertainty in the lignin hypothesis, and I'd assume that our knowledge of land biohistory is far more detailed than what we know about the oceans), lots of science to be done!
They're literally terms to describe dietary habits, nothing else. A human who eats meat is a carnivore, regardless of whether they're hunting live animals or being served chicken nuggets on a plate - the only behaviour that's relevant is whether the diet includes meat or not.
The title is unnecessarily dramatic.
Looked up the meaning and the roots of the word. Feel compelled to mention that the etymology is pretty wild.
hadal: “of or relating to the deepest ocean; beyond 6000 meters” ← Hades, Greek god of the underworld ← ᾍδης (Hā́idēs)
which is
‘Perhaps from Proto-Indo-European *n̥- (“not”)+ *weyd- (“see”) + *-ēs (“adjectival suffix”), literally “that which is unseen”’
or
← ‘from *sm̥weyd-(from *sm̥- (compounding stem) + *weyd-(“see”) + *-ēs (“adjectival suffix”), literally “see-together” or “uniter”)’
… according to Wiktionary: https://en.m.wiktionary.org/wiki/ᾍδης
That’ll be one exploding-head emoji for me please.