Villains who can actually serve very important purposes and need to be kept alive despite their evil deeds. No matter how sadistic, bloodthirsty, cruel, or even how much they are hated, these villains have to live or something terrible will happen. However, a) once they fulfilled their purposes or if there is a good / less evil alternative and b) they still choose not to redeem themselves, whether they get killed off or suffer a demise no longer matters, as they are no longer needed at that point and still have technically done evil things.

These characters are similar to Virtually Resourceful characters, but are obviously opposed to their alignment. Also, these characters are usually created to keep the story relatively three-dimensional.

Villains who qualify[]

Villains who fall under at least one criteria are Necessary Evil:

1. Some anti-villains and/or extremists with the intent to help the hero or to save a large number of people. Often, these characters are considered anti-heroes (a noteworthy example is The Punisher, while being a violent vigilante, fights off various supervillains, defeats them and steals their resources).
2. Villains who are keeping a much worse evil at bay (e.g. Galactus keeps Abraxas, a being able to destroy entire universes, out of the Marvel Universe; Anna Morgan, whilst depicted as an abusive and mentally disturbed mother, keeps her adoptive daughter Samara Morgan from continuing her torment and killing spree; The SCP Foundation keeps thousands of dangerous and world-ending anomalies in containment to protect the human race).
3. Abusive employees and employers that actually are capable of doing their jobs and it's made very clear the business cannot succeed without them (e.g. Benson is heavily trusted with keeping the park afloat; Malcolm Tucker was very capable in doing his job as the Director of Communications).
4. Forces of Nature who exist as a fundamental aspect of the world of a given franchise, and whose existence is only antagonistic out of necessity or to resolve some other issue with the wider world around them (e.g. Multiple interpitation of Death while somtime they going too far in their method, still need to exist to prevent overpopulation).
5. Villains preventing catastrophes, including the end of the world or universe, with their mere presence or actions (e.g. Kyubey prevents Armageddon with energy collected from magic girls; the SOUL is needed to seal the Dark Fountains to prevent the Roaring, which would result in the Titans destroying the world; The Chairman keeps the Frightmares in the Flow of Failed Phantoms for the safety of both human and ghost worlds).
6. Evil vs. Evil scenarios where one of the villains is the only subject strong/powerful enough to defeat the more dangerous threat (e.g. Lord Garmadon was the only person who could wield all four golden weapons to destroy the Great Devourer; Harry Sparks and his friends were the only ones who could stop Gil Yepes from destroying the Killian University after his betrayal towards Liza Everton and Jimbo Farrar, Stewie Griffin was the only one who could stop Diane Simmons from killing Lois; Heinz Doofenshmirtz was the only person to stop Aloyse Von Roddenstein from creating a new Ice Age).
7. Unlawful businesspeople that can provide a product or service that is needed or well-liked by people (e.g. Mr. Krabs runs the Krusty Krab which produces the very well-accepted Krabby Patty; Seto Kaiba is in charge of KaibaCorp). This can also apply to entertainers who perform or create to please and entertain people (e.g. Misa Amane).
8. Villains performing villainous acts that lead to beneficial results/consequences. It can be either:
   1. Those who takes up Stealth Mentor path where they deliberately focusing on antagonizing the hero and those around them in hopes of shaping them into legitimate protectors. (The Architect created the One prophecy for Neo and his predecessors and Kessler trained his younger self Cole MacGrath so he can better prepared in facing The Beast).
   2. Those who humbling the hero or other heroic characters by forcing them to see their flaws and other errors, often through extreme methods (e.g. Mister Babadook made Amelia comes to terms with her husband's death).
   3. Villains who made a certain mistake which led one of their victims becoming the very hero who will eventually stand up to stop their evil. In some cases, said mistake counts as a self-fulfilling prophecy (e.g. Joe Chill murdering Martha and Thomas Wayne leads to Bruce Wayne becoming Batman, who then fights against criminals and worse villains like him; Shocker turned Takeshi Hongo into the first Kamen Rider, giving him the power to rebel and fight against them).
   4. Villains who inadvertently provide opportunities for the heroes to exercise their heroic skills and abilities or facilitate beneficial outcomes for society, precisely because of their heinous and disruptive actions (e.g. Ke-Pa has ravaged the Valley of Peace, so when he was defeated, Master Oogway created the Jade Palace to make sure Ke-Pa's plans can't be fulfilled if he returns, where the main protagonist, Po, also the Dragon Warrior, trained to be a better hero and that resulted him into defeating villains like Tai Lung, Lord Shen and Kai; Satan crucifying Jesus led to the latter atoning for the sins of humanity, thus bringing salvation, which could only be achieved through death).
9. Wealthy villains that helped the city they live in (e.g. Mr. Burns is Springfield's only taxpayer and one of the owners of their nuclear power plant, and Lex Luthor, who employs most of Metropolis' workforce.)
10. Family breadwinners (e.g. Peter Griffin, while being an arrogant dimwit that causes many problems to his relatives, is often the only person in the family who actually works to put food on the table and therefore, his family must tolerate his actions as he is the only one capable of providing for them; Walter White entered the drug business mainly to provide enough money for his family before he dies from cancer).
11. Villains with important information that are necessary for the heroes and the other characters to either save people or defeat more dangerous evil-doers. (e.g. Hannibal Lecter had information that helped Clarice defeat Buffalo Bill).
12. Villains who must be kept alive so that no bad consequences would follow because of their untimely deaths (e.g. the Riddler gave Batman vital information about the Pact and had Tiffany Fox not shot him dead, it would've caused much less problems for the GCPD, the Agency, and even Batman himself, as the Agency would've used him to find S.A.N.C.T.U.S. as a bonus and then left Gotham City behind; also, Amanda Waller wouldn't have demoted James Gordon from being Commissioner nor fired him from his job, and Gordon wouldn't have had Bruce Wayne arrested because he wouldn't have been working with the Pact while being undercover for the Agency because the Riddler would've filled in that position instead).
13. Fallen Heroes whose fall into villainy will bring undesirable consequences had it ever made known to public due to their formerly benevolent reputation (e.g. when Harvey Dent helped locked half of Gotham's crime bosses away in The Dark Knight, he gave hope to Gotham; but when he became Two-Face and started killing people, Gordon and Batman knew if the truth came out, people would lose hope so Batman willingly took the blame so Dent would stay a hero after his death).
14. Villains from the Lawful Neutral alignment who keep order, and it's obvious that situation would be worse had they not been present (e.g. Razoul, despite being amoral and corrupt, keep peace and order in their home cities).
15. Hostile biological species, tribes, or civilizations that, while hostile or xenophobic, are capable of producing children and families. Killing of a naturally hostile race, civilization or tribe would still be considered an act of genocide and collective punishment especially if some members, particularly children, are innocent. (e.g. the Neimoidians, the Grox, and the Predators).
16. Evil teachers who keep unruly students in line and /or are the only ones willing to enforce discipline.
17. Alter-Egos or powerful entities within a hero, the ones that has possessed the hero, or grant them their powers, resulting in them saving the hero from time to time (e.g. Zangetsu possessed Ichigo multiple times and, as a result, saved his life multiple times; Kurama sometimes possesses Naruto or gives some of his power to prevent Naruto from getting killed or to fight off a powerful foe).
18. Former villains that joined, or at the very least, helped the heroes (e.g. Yasha, Trish, Bagramon when he was the Old Clock Shop Man, and many others).
19. Villains who saved a protagonist(s), attempt to sacrifice themselves to save a protagonist(s), or sacrificed their lives to save the protagonist(s), such as SA-X, Sybok from Star Trek V: The Final Frontier; Android 16 who attempted to self-destruct to kill Cell and later encouraged Gohan to unleash his anger before Cell killed him; Crosshair who saved Omega and AZI from drowning because they saved him from suffocating in the first place, and questionably Nemesis from Resident Evil: Apocalypse.
20. Double agents that are on the side of good and pretend to be evil. These people can be a friend of a Hero who was originally at odds with heroes but ultimately chooses to switch sides and leak the antagonists' evil scheme on them (a famous example is Severus Snape, who pretended to be allies with Lord Voldemort and his forces but was secretly helping Dumbledore and Harry Potter). This can also apply even if they spy only for personal gain and not because they truly want to redeem themselves (e.g. General Hux gave information to the Resistance on Emperor Palpatine's return and saved Finn, Poe Dameron and Chewbacca from execution, although only for an advantage over Kylo Ren in their power struggle).
21. Dark Forms, Devils, Gods of Evil or other personifications of evil that are literally the source of evil. If they are destroyed, evil would cease to exist, and life would no longer be able to feel sadness, anger, or any other negative or evil emotion and be forced to feel positive all the time, be incapable of doing anything evil or negative, and think that negativity is unnatural. So villains who are the source of evil have to be kept despite their evil influence. Additionally, they might be preserved because their existence is what keeps the lives of the protagonists meaningful (e.g. The Overlord, as the source of all darkness, must remain alive so that the balance can remain stable).
22. Monotheistic deites in which their presence is linked to the universe that they created and if they die, their world will no longer exist and all the life in the world gets completely destroyed. This goes to all the villainous Supreme Beings because even if they are evil, they must be always active.
23. In video games, certain evil creatures can be farmed for useful items (killing a Wither will give you a Nether star, which is the key ingredient in making a beacon).