Sun Wukong from the novel Journey to the West, when it comes to dealing with demons and bandits, who he sees as evil monsters who prey on the weak (especially those who want to eat Xuanzang). This is most notably seen during the White Bone Demon and the Doppelgänger chapters. In some adaptions, Xuanzong kicks Wukong out not because what he did (like killing innocent humans who were all actually demons in disguise or groups of bandits), but because of his Knight Templarish attitude.
Jorge of Burgos from Umberto Eco's The Name of the Rose is the epitome of this trope. He's already a crazed extremist at the beginning of the book, and then he kills - or has killed - seven people. All to prevent somebody from reading a book that Jorge considers heretical: Aristotle's lost chapter of the Poetics discussing comedy. His Karmic Death comes as a huge relief, even to his allies.
The Anathemata Curialis from the Felix Castor series will do anything to fight back the suddenly rising wave of undead and demons, including, yes, recruiting them. You'd just better hope that you don't get possessed by anything, or otherwise get in their way.
In Harry Potter, Dumbledore and Grindelwald once wanted to take over the world, so wizards could stop hiding: "Muggles forced into subservience. We wizards triumphant. [Us], the glorious young leaders of the revolution." At the time, Dumbledore stressed that they should use as little force as possible and that with their power Comes Great Responsibility. Although Grindelwald may have seemed to be one during his rise to power (he even used the slogan "for the greater good") Dumbledore suspects he was only hungry for power.
Voldemort himself (among others) believes, not that he is a "good guy" (according to him, "there is no good or evil"), but that wizardkind must be purged of what "infects" it - namely, everyone who's not a pureblood or half-blood wizard. He's not a perfect example though: there appear to be some daddy issues at the heart of his motivations.
His ancestor, Slytherin, seems to have based his prejudice towards muggles and muggleborns on a belief that they are inherently untrustworthy, and some of the secondary villains (namely, Bellatrix and Umbridge) do have a moral belief that all non-purebloods are dangerous subhumans.
Another Knight Templar is Barty Crouch Senior, who authorized the Aurors to use unforgivable curses and sent people to Azkaban without trial, and when he did give trials, they were often for show in a Kangaroo Court.
The Whitecloaks in The Wheel of Time series. They think that all Aes Sedai are servants of the Dark Lord and get neighbors to accuse each other of being "darkfriends". Only three of them are presented as honorable men.
The Questioners, a branch that specializes in tortured confessions, are so nuts that not even the Whitecloaks can stand them.
Also from Wheel, the Red Ajah, that faction of the Aes Sedai dedicated to finding and depowering men who can channel. A large portion of them began to despise all men (they don't even have Warders), and eventually, they began to break Tower Law by gentling men outside the tower. As punishment, the three sitters were all exiled to farms.
In Jim Butcher's The Dresden Files novels and, to some degree, the TV series based on them, the White Council and their enforcers, the Wardens, frequently come across as Knights Templar, primarily with their draconian enforcement of the Seven Laws of Magic (usually entailing instant beheading). Morgan, one of the leaders of the Wardens, is the first and best example of this. However, this ends up somewhat subverted later in the novels when Harry, who had been viewed as a troublemaker at best and Lawbreaker at worst, and had once been under a one-strike-you're-dead parole, is recruited into the Wardens after many of them are slaughtered during the war with the vampire Red Court. Even Morgan changes his position: while he still believes that Harry is dangerous, he no longer thinks that he's evil - just arrogant, undisciplined, and stupid. This is a major change from a character previously thought to be unchanging. Even more of a subversion, as Morgan's changed opinion is mostly right.
The Wardens do end up seeming less like Knight Templars later into the series, once Dresden becomes a Warden and has to face a lot of the same situations.Breaking the Laws of Magicwarps and corrupts people's souls, and bringing someone back from the edge of corruption is a long and risky process. Harry's ex-warlock apprentice, Molly, only broke the Laws of Magic twice, and backslides repeatedly despite Harry's constant supervision. Once Harry can't supervise her any more, she starts building up a body count.
In the Revelation Space universe, there is a species which plans to prevent any technological civilization from arising for 6 billion years to make sure that life can flourish after the Andromeda and Milky Way galaxies collide. They are perfectly willing to kill trillions of sapient beings and wipe out whole species in order to achieve this goal.
A similar feature is in Anvil of Stars and its prequel, where the good guys' mission is to defeat a group of planet destroyers who eliminated Earth. This involves eradicating nine different intelligent species who have the misfortune to be in the way. Apparently, those races were deliberately created as (non)human shields.
In certain time periods of Larry Niven's Known Space universe, minor crimes such as "repeated traffic violations" are punished by execution. However, this is primarily because the organs of all executed criminals are harvested for the "organ banks" for use in transplants.
In His Dark Materials, while Lord Asriel's grand plan is certainly noble in theory, the fact remains that he first kills a child in order to open the first portal to the parallel worlds, and then pulls a Lucifer and makes war on Heaven, all in a plot to kill God. We're not too sure if it's a good thing or a bad thing that he succeeds. Asriel is still pretty awesome.
This trope also appears in The Amber Spyglass, in which the Magisterium dispatches a young priest named Father Gomez to kill Lyra. Gomez fairly blazes with righteous piety and the belief that he is on a holy mission. The sin of murder is explained away by the church as having already been "paid for", since Gomez has been doing pre-emptive penance for most of his life. It would be Artistic License - Religion if the story wasn't set in a parallel universe (Dante has a very special place in his Inferno for people who apply this sort of logic). Pre-emptive penance requires a logical impossibility; you can't repent of a sin in advance (and no penance is effective without repentance) because true repentance means wanting to have never done the sin. Truly repenting in advance would mean that you'd never do the sin.
In the Dragonlance series of books, the Kingpriest is a Knight Templar. His insistence on destroying all evil leads to him attacking neutral people and gods (because if you aren't with us, you are against us) as well as evil. His upsetting the balance, as well as demanding from the gods the power to destroy all evil, brings about the destruction of a large part of the planet, as all the gods decide that humans have gone too far and get pissed.
Forgotten Realms is ripe with these. Khelben "Blackstaff" Arunsun is a great self-sacrificing hero when there's a real threat. When there isn't, however, he may easily find something or someone to absurdly overreact to. Eventually, he got kicked out from Harpers for carrying "dealing with evil forces against other evil forces" idea too far for their taste (and they aren't quite paladins themselves). That's the mild case.
Renwick Caradoon, co-founder of The Knights of Samular, used his niece as a bait for a Deal with the Devil — or rather, an incubus — whom he planned to betray. When this backfired, he locked the fiend up...along with about 200 relatively innocent souls. When his sanctimonious indiscretions and half truths sent Khelben into seething rage, he ensured that the power acquired from the deal stays with him — through blackmail and hiding behind the paladins' With Us or Against Us mentality — and continued in the spirit of such deeds.
"Evil!" the sword hissed, jerking Sir Thongolor's arms this way and that. "Any who would resist or prevent me or the holy warrior who bears me must be evil — and must be destroyed!"
Inspector Javert from Les Misérables. To him, law is everything, and when he realizes that to act lawfully is to act unethically, he snaps and commits suicide. Doubles as a InspectorJavert.
Javert's ideal, was not to be human, to be grand, to be sublime; it was to be irreproachable.
Villains by Necessity by Eve Forward portrays a whole world dominated by Knight Templars, in a quite serious inversion of many classical tropes.
The Humanoids, a model of robot from Jack Williamson's 1947 novella "With Folded Hands..." (and the follow-up novels The Humanoids and The Humanoid Touch) are classic examples of this trope combined with the Literal Genie trope. The Humanoids are programmed to "Serve, Obey, and Guard Men from Harm". Since nearly every human activity has some risk of harm associated with it, the Humanoids, in practice, never let anyone do anything (although, occasionally, if they really need a single human's help to "protect" a great many humans, they will bribe them with limited autonomy). When people begin to complain that these restrictions are psychologically harmful, the Humanoids drug or lobotomize them. In the end, the Humanoids invent a machine that gives them Psychic Powers and use it to institute an Assimilation Plot).
In the Ciaphas Cainnote HERO OF THE IMPERIUM novel Duty Calls, an Inquisitor is willing to stage a massacre, abandon innocents (including children) to an alien attack, and actively cause an alien attack and massacre (and trying to assassinate Cain three times) on the grounds that what he is protecting is too valuable for the information to get out. He even thinks that Cain will agree with these actions because of the importance of the artifact.
In the same book, Battle Sisters refuse to retreat to the line of their defenses because they must serve the Emperor; Cain finally points out that if the Tyranids outflank them, they will be responsible for the massacre of the civilians in the Emperor's Temple. This not only persuades them to retreat, it causes one of them to thank him later, for reminding them of their duty, and admit that their zeal had lead them astray. Later, this takes on a grimmer note. The Sisters realize that they have sheltered a renegade inquisitor. Even his deception does not ease their guilt; their zeal had blinded them to the facts. In atonement, they sacrifice their lives to ensure the escape of the Inquisitor who told them the truth and her party.
In Xenos, Inquisitor Gregor Eisenhorn explains some of the mindset behind such people. He finds himself in a position where he can either mercy-kill an innocent and allow a heretic and traitor to escape, or follow the heretic and allow the innocent to die horribly. He talks to the reader for a moment, saying that, if you would kill the victim, that is good, you are human. On the other hand, you are not an inquisitor; he must place the millions of lives that the heretic threatens over the life of one. If one man must die, that millions might live, that is how it must be.
Many people in a position of authority in the Imperium are Knights Templar — the difference is that they have no real choice. Warhammer 40,000 is set in an incomprehensibly horrible dystopia. The Imperium can't afford to err on the side of mercy — Chaos is just too dangerous, and spreads too fast. Killing a thousand people just to nail one heretic or rogue psyker may be extreme, but it has been long-established that the whole galaxy would be overrun by Chaos if they did anything else. Chaos runs on fear, suffering, and insanity, so by their actions the leaders of the Imperium are ensuring that they can never truly defeat it. They may even be feeding it and making it stronger.
In the Sword of Truth series, Sister Nicci fits the trope better than anyone else. Even after her Heel-Face Turn, she keeps some definite traits of this.
Lucas de Beaumanoir in Sir Walter Scott's Ivanhoe, who actually is a Knight Templar — indeed, the Grand Master of the Order. Although most of the Templars in the novel are corrupt and immoral, Beaumanoir "is of a different stamp — hating sensuality, despising treasure, and pressing forward to that which they call the crown of martyrdom..." He comes to the preceptory of Templestowe to root out vice and, in the process, puts the noble Jewess Rebecca on trial for sorcery.
Bois-Guilbert is a subversion; his main vices are lust, and arrogance and the only time he gets close to fanaticism was in his rather creepy protectiveness toward Rebecca after attempting to rape her. The Grand Master of the Templars though is a straight example.
Maxim, an uninitiated Light One in Sergey Lukyanenko's Night Watch, is this. He has been enchanted so that he can sense the evil of the Dark Ones but not the good of the Light Ones, causing him to consider himself a lonely crusader in a world choked with evil (as opposed to a world in an eternal stalemate between evil and good), leading him to kill low-powered and not particularly evil Dark Ones. Actually, given the Night Watch's philosophy of good as working for the greater good, the entire side of light can occasionally become Knights Templar, and we are explicitly told that both Soviet Communism and Nazism started out as plots by the Light to win against the Darkness.
Terry Pratchett's Discworld Auditors are very much this. They just want a universe of perfect order. Is it their fault that life, especially intelligent life, keeps getting in the way of this?
The Calvarians of The Reynard Cycle view all outsiders as inferior at best, and subhuman monsters at worst. (It doesn't help that some of them actually are subhuman monsters.) Subsequently, they see themselves as heroic warriors battling the forces of darkness, and don't bat an eye when slaughtering and/or enslaving foreigners. This ideology is enforced by the blood-guard, a State Sec whose function is to ferret out dissent and quash unlicensed breeding. Even high ranking Calvarian generals have one constantly looking over their shoulders.
In Outbound Flight, Jedi Master Jorus C'baoth is renowned for cutting past the bantha poodoo and solving whatever he's been assigned to solve very quickly. He also believes that, as a Jedi connected to The Force that binds all things, he's under the best leader imaginable. Non-Jedi, if they don't have Force Sensitivity, are to submit - well, everyone is to submit, but his Padawan watches him striding through a crowd which makes way "like a swirl of dried leaves" and realizes that she's starting to think of them the way he does. In command of Outbound Flight, he's terribly authoritarian and controlling; slowly, every decision becomes his decision, and his decisions are always right. C'baoth is largely responsible for Outbound Flight's destruction. The other Jedi on board questioned what he was doing, but...he was C'baoth. Surely, it couldn't be as bad as it looked.
Jorus C'baoth had a clone, Joruus C'baoth, in The Thrawn Trilogy. The clone had each of those traits, but stronger and with a dash of crazy. Joruus went so far as to use the Force to control the people he led and guided.
In The Lord of the Rings, it seems that Gandalf would have become some sort of Knight Templar had he taken the Ring.
"Gandalf as Ring-Lord would have been far worse than Sauron. He would have remained 'righteous', but self-righteous. He would have continued to rule and order things for 'good', and the benefit of his subjects according to his wisdom (which was and would have remained great)." (Letter 246)
And Gandalf recognizes this, as he explains to Frodo after he tries to give the Ring to the wizard. "Understand. I would use this ring out of a desire to do good. But through me, it would wield a power too great and terrible to imagine!" This is a temptation for Galadriel as well - both Sam and Frodo encourage her to take the Ring, insisting that she'd help people and do good with it. She responds sadly that that is only how it would begin...
Stannis Baratheon from A Song of Ice and Fire is a good example of why a truly just man is terrifying. He gets engaged in the Succession Crisis not because he actually wants to be king (he really doesn't), but because he feels that since he is the nearest successor, he has to. When dealing with a smuggler who brought him supplies during a siege, he knights him for his valor and chops off the fingertips of his left hand for his lifetime of smuggling. By the time of the story, he has actually become less of a Knight Templar and is willing to compromise his justice for the sake of his ambition — most notably, murdering his own brother and letting others take the fall, trying to bribe the Lord Commander of the Night's Watch and have him forsake his oath, and not punishing those who sided with his younger brother, Renly, when he has a better claim to the throne. The smuggler in question — Davos — longs for the days when Stannis was a proper Knight Templar, because he no longer recognises this man who has betrayed so many of his own principles, and is willing to commit horrible crimes in the name of his ambition.
The main advisor for Stannis, Melisandre of Asshai, is a perfectly straight example. A religious fanatic willing to murder, commit human sacrifice, and kill children for the sake of her goals, she firmly believes every action she does is right and for the greater good, and thus any method she uses is justified. She gives a lecture to Davos at one point where she declares that everything is black and white, good and evil, right and wrong, no exceptions. At one point she asks Davos whether he is a good man or not. Davos answers humbly by saying that he is both good and bad, as he has done good and bad things. Mel replies that such a thing is impossible. "An onion can be clean or rotten. If it has any rot in it, then it is a rotten onion."
Keira in Sandy Mitchell's Dark Heresy novels Scourge the Heretic and Innocence Proves Nothing - fanatic, dedicated to eradicating evil, convinced of the heinousness of the most minor of faults, and finding Dirty Business whenever she has to pass some trivial evil by. And people who have known her in the past think that she's mellowed out like this. Convinced that Sex Is Evil, she's first oblivious to and then deeply disturbed by the notion that she is attracted to a man, even though they are both free to marry. To be just, confronted with a prostitute trying to escape that life, she is awed by the effort the woman put into her escape.
The Silence in Laura Anne Gilman's Retriever series began as an organization dedicated to protecting Nulls from supernatural dangers. Motive Decay led to them deciding that the best way to do this is to destroy all supernatural beings, including sentient non-humans and magic wielding humans.
On one hand, the Knights of Khryl in Caine Black Knife count as Knights In Shining Armor, almost to a man. Almost. They also oppress nonhuman species, are completely inflexible, and are led by a woman who believes herself to be, in her own words, "incapable of sin". Since Khryl is a hardass, they do have some supernatural justification.
In the Dale Brown novel Edge of Battle, the US finds itself on the slippery slope to this as US-Mexico tensions grow due to the villain's plot, but ultimately avoids dropping off the slope.
"First of all, what is it really all about? What is it you object to? You want to abolish Government?" "To abolish God!" said Gregory, opening the eyes of a fanatic. "We do not only want to upset a few despotisms and police regulations; that sort of anarchism does exist, but it is a mere branch of the Nonconformists. We dig deeper and we blow you higher. We wish to deny all those arbitrary distinctions of vice and virtue, honour and treachery, upon which mere rebels base themselves.
The scriptures of the Church of God Awakening in the Safehold novels explicitly state that "Extremism in the pursuit of godliness can never be a sin". The Inquisition considers this a license to commit any atrocities necessary to secure their power base...in the name of God.
In Death: Donald Dukes from Purity In Death is very much this. He actually believes that he is so right that he has to murder every person who does not fit in his warped vision of the world.
Hollyleaf from Warrior Cats reaches this state at the end of the third arc, becoming obsessed with order to the point of attempting to kill her own mother for having an affair with Crowfeather. Later on, she has a Heel Realization while hiding by herself and returns much more sane and mature.
Averted by the templars themselves; they disapprove of Dragonetz' mill but do not try to kill him over it.
A straight example is Raymond de Toulose who dresses like a monk, believes his goals are the same as God's goals and thus fully good and righteous. Others think of him as 'the devil' or 'the demon' because of his cruelty.
The government in A Clockwork Orange, when pushing the Ludovico treatment in an attempt to reign in crime. When confronted by the prison chaplain that the treatment takes away a person's choice to willfully be good, the Minister of the Interior replies that they're "not concerned with motives, with the higher ethics".
The Commonwealth Police in Linda Nagata's The Bohr Maker, particularly Kristin, it's Chief who is willing to terrorize, torture and even kill innocents, subvert legal procedure and at the end nuke a colony of thousands to protect her vision of Nature's "purity".