Hungry Menace

They commit acts of evil. They kill, torture and rape. Why? Not for fun, not for profit, not because they hold a grudge against their victims. Instead, their very existence and survival depends on it. Maybe they actually need to eat some specific food like vampires, maybe they feed on your pain, maybe some supernatural curse demands human sacrifices from them in exchange for sparing their life.

This, similar to introducing Always Chaotic Evil people, is an easy way to create conflict that can only be resolved with violence, as you can't negotiate with someone or something who need to kill you to survive. A Hungry Menace may be portrayed in different ways -- from the horrible abomination that must be slain for the good of everyone to a Non-Malicious Monster or even a natural part of the universe that is not evil at all.

If such a being kills and tortures to extend his life beyond its natural span, it's Immortality Immorality. It is close, but different as it present not doing amoral things and dying as the natural and only good solution for this problem, while this trope is more ambiguous. See also; Horror Hunger, Face Monster Turn and Phlebotinum Muncher. For a typical hero response, see Guilt-Free Extermination War.

General

 * Vampires have to drink human blood, or they will starve and die. If the vampire is portrayed sympathetically, he will cheat and drink animal, donated or synthetic blood, or else he may be pickier than others about his prey.
 * Romero-esque Zombies are just driven to eat flesh, though for other zombies, it's our brains that they're after.

Anime and Manga

 * Bleach has Hollows, the hate-filled souls that survive by either eating other souls or, should they be advanced enough, cannibalize other Hollows. It's only past this trope if the Hollows keep personalities(take Shrieker, who used a boy's soul to hunt those he was owned by((long story))). So far, though, the Arrancars' diets are unknown, but all the ones, with their past revealed when they were hollows ate others.

Comic Books

 * While he might not hold any malice towards his victims and supposedly serves a few "necessary functions" within the universe, none of that changes the fact that Galactus, Devourer of Worlds has a nigh-countless number of genocides to his name.

Film

 * Played with regarding zombies in Survival of the Dead.
 * The Blob.
 * The Gyaos in the Gamera films survive by feeding on human flesh.
 * This is subverted with Irys. We discover early-on in the film that it has a horrific way of feeding (IE: using the claws on its arms to impale and then drain the life-force of other creatures), but, its main goal is really
 * Baragon from the Godzilla films.

Literature

 * The villain in CM Kornbluth's short story "The Mindworm".

Live Action TV

 * Fringe had a Monster of the Week who harvested prostitutes for hormones which will stall his improbably rapid aging.
 * Bainlings from Legend of the Seeker made a deal with the Big Bad and had to kill people or they will be dragged back into afterlife.
 * Wraith can only eat humans or other Wraith. Or they will starve to death.
 * The Energy Beings in the Star Trek the Original Series episodes "Day of the Dove" and "Wolf in the Fold."
 * Eugene Tooms from two episodes of The X-Files is stated to need human livers to survive.
 * And Rob Robertson from a later X-Files episode needed brains... not that he didn't try to live as a non-mutant.
 * The bone-eating monster in the Farscape episode "Bone to be Wild" is Actually Friendly, just very very hungry.

Video Game

 * Amnesia the Dark Descent protagonist  in order to escape inevitable death from curse

Tabletop RPG

 * The Horrors in Shadowrun and Earthdawn.
 * Dungeons and Dragons
 * The Darkhood monster in the Mystara setting.
 * The Nightmare Court in the Ravenloft setting's Nightmare Lands.
 * The Gleft in the Dying Earth RPG supplement Scaum Valley Gazetteer.
 * Warhammer40000. The Dark Eldar inflict pain, misery, and destruction and literally get younger to achieve a degree of eternal life. Haemonculus need to do this in a twisted, creative fashion on a daily basis or they'd wither away. Wyches perform gladiatorial fights on captured prey that the Dark Eldar attend, their faces and skin noticeably younger and healthier than when they first went in.

Theater

 * The Gilbert and Sullivan play Ruddigore (1887). Centuries ago, the first Baronet of Ruddigore persecuted witches. As she was burnt at the stake, one of his victims cursed all future Baronets of Ruddigore to commit a crime every day, or die in agony. Every Baronet of Ruddigore since then has died when he couldn't bring himself to continue a life of crime. Luckily the current Baronet discovers a Curse Escape Clause.

Real Life

 * Carnivores need to eat meat. Some carnivores are scavengers who primarily feed on the dead, but others are predators who have to kill other animals in order to survive.