About Demon Hunters
Demon hunters are dark, shadowy warriors who are shunned by the greater night elf society. They made a pact, long ago, to fight against the forces of chaos using its own terrible powers against it. These mysterious warriors ritually blind themselves so that they develop spectral sight that enables them to see demons and undead with greater clarity. They wield demonically charged warblades in battle and even call upon demonic energies to augment their formidable combat skills. Although they are counted as some of the mightiest warriors within the night elves' society, the demon hunters are always maligned and misunderstood for making their selfless pact with darkness.
-Warcraft III Game Manual p.118
Demon hunters are skilled warriors who have pledged their lives to fighting evil by using its own powers against it. Though many among the night elves and the Alliance fear the demon hunters and their pact with darkness, all know that the demon hunters and their demonically-charged warblades have played an important part in combating what remains of the Burning Legion.
-Alliance and Horde Compendium, Warcraft RPG p.27
Demon Hunter NPCs
Demon hunters are dark and shadowy warriors. Illidan Stormrage is the most infamous demon hunter, and was the first of their kind. Even Illidanís own brother did not appreciate his sacrifice, and locked him below ground for thousands of years for trafficking with dark powers.
Illidanís case was unique. Sargeras, lord of the Burning Legion, burned away Illidanís eyes with magic fire until only scorched sockets remained. Illidanís resulting sight was a maddening display of violent colors. His altered vision allowed him to easily recognize both demons and mortals with magic powers. In addition, Sargeras covered Illidanís body with black tattoos that increased his arcane power.
Later, a group of night elves, inspired by Illidanís example, made a pact to turn the Burning Legionís powers against it, fighting destruction with destruction. Obviously they could not gain their powers in the same way Illidan did, but they discovered other means. In the millennia since, other night elves, and a few creatures of other races, have made the same pact, binding demonic essence in their bodies and using it to destroy the Legionís minions.
Demon hunters have a variety of abilities that assist them in destroying demons, though the power coursing through their bodies also allows them to prevail over lesser foes. Demon hunters eschew heavy armor, valuing mobility and speed.
-Alliance Player's Guide, World of Warcraft RPG pp.47-48
The above excerpts do a pretty good job of covering the "core" of a demon hunter. It's important to note that nowhere does it claim them to be "power-hungry villains bent on destroying everything they hold dear." No, demon hunters, at their very core, seek to destroy the Burning Legion. Their methods are extreme in the eyes of those who do not understand, and their sanity is definitely in question, but a demon hunter is not a template for a cookie-cutter villain.
Demon hunters make a noble sacrifice. Not only are they sacrificing what remains of their mortal lives to mistrust and potential corruption, but they are also sacrificing any potential afterlife they may have believed in. They bind themselves to demonic forces, demonic essences, and in some cases to demonic spirits entirely. Some demon hunters even "evolve" so far as to become demons themselves (Illidan Stormrage is the seminal example). This means that whatever hope of a peaceful afterlife a demon hunter may have is effectively obliterated under the reality that they're likely to be sucked into the Twisting Nether with the rest of the demon spirits.
Can YOU fathom such a sacrifice? If you were given the opportunity to fight evil using evil's powers, but you were given no guarantee that you'd succeed, and if you were told that you'd be shunned and mistrusted for the rest of your life AND that your spirit would be sent to your world's analogue of Hell...would YOU choose that path quickly? Demon hunters do.
It is true that many crave power. This much is told to us through the questlines dealing with Varedis and Illidan's school of blood elf demon hunters atop Karabor in Outland. But even these demon hunters seek to fight the Legion. Illidan has surrounded himself with demons of all sorts who are in opposition to the Burning Legion. And even though his methods are questionable, his original intent still holds.
It is also revealed to us that the power hungry demon hunter trainees atop Karabor are all expected to die. The rituals and training are harsh, and only those with the sheer will, ability, and conviction to survive make it to the end.
Corruption and madness are not uncommon amongst those who walk the path, but it is important to understand that these things do not have to be the ONLY thing that defines a demon hunter. There is a noblity in the path that they walk. Their sacrifice is immense, and they receive nothing but pain and suffering in return for it. Because of this, true demon hunter characters become an exceedingly complex mix of motives and emotions.
THAT is what makes a demon hunter.