Some chapters of the Imperial Space Marines themselves fall into this. There's even a chapter called the Black Templars who are notorious for their intolerance of aliens, mutants, and heretics — going as far as to "purge" entire planetary colonies because somebody there has bought equipment from alien traders. (The standard Imperial response would be to round up and execute only the buyer/s and whoever is suspected of being a collaborater/enabler, unless the stuff came from a Rogue Trader — in which case it's all good, unless it's contraband even for Rogue Traders — or starts showing signs of Chaos taint.) Their colours and insignia are derived from The Knights Hospitallers, though.
This applies to the Tau, too, to a lesser extent. At least they ask others to join them first, and if they don't they open fire with everything they got.
If we ignore the extreme speciesism common to all factions in the universe, the Eldar would come across as almost heroic. As it is, the whole "We would rather ten thousand humans die than one of our own" mindset dooms them to this territory.
In the RPG Deadlands: Hell on Earth, there is a group of people - including some player characters - that call themselves Templars. In a post-apocalyptic setting named, well, Hell on Earth, you can imagine what they're like: unflinchingly hard-nosed and often turning away those in need simply because they don't live up to some subjective moral standard. Even worse, sometimes they force others, who are less "worthy", to do unseemly tasks with their awesome supernatural powers. And they're the good guys. They were founded by a man who witnessed how the Law Dogs, a group of people who try to be paladins who save everyone, despite lacking magic, often end up dead or betrayed by protecting "innocents" who weren't worth the trouble, and they receive their divine powers because the Almighty, who originally empowered the more actively benevolent Blessed, has taken a harsher stance towards humanity since the Reckoners escaped.
Jo, Templar Grand-Master: Here's the way we see it. The old world was full of greedy, violent people. It was also full of lazy bags of crap who knew the world was going to Hell and didn't do a damn thing about it. The small minority who stood up against evil, who sacrificed everything to help fight oppression, didn't usually get much help. Templars have vowed not to let that happen again.
How harsh are the Templars? There's a Templar archetype in the core rulebook for Hell on Earth. In its attendant fluff-bite, the Templar lambasts a town as to why he considers them unworthy of saving from a bandit gang, and declares he would normally leave them to the bandits' mercy. However, since there is another town nearby that is also in the bandits' path who he does consider worthy, he decides to be merciful... He's sending every man and woman in town on a Suicide Mission against the bandit's camp, whilst their children are being sent to the other town as slave laborers to help ensure they get their defenses ready in time, just in case any of the bandits survive fighting his Cannon Fodder.
At least 10% of anyone involved in anything in Eberron are Knights Templar. The Church of the Silver Flame's hardliners a) want to forcibly convert everyone and b) consider fire a divinely sanctioned weapon, even against civilians. This is from a Lawful Good religion, with help from Eberron removing the rule about cleric alignments. And one of the basic premises of the Silver Flame is that swordpoint conversions are utterly meaningless. Shifters are also rarely members of the Church, mainly because of their recent crusade against lycanthropes: many shifters were similar enough to the lycanthropes to be targeted as well. They're still not pleased on that score. Then there are the Ashbound, an Animal Wrongs Group of the first order whose ultimate goal is the extermination of everything...unnatural, including arcane magic, constructs (usually including warforged), divine magic not tied to druidic traditions, the lightning rail, most manifest zones, cities, most kinds of house and any druidic sect not wholeheartedly devoted to the scouring. The government of the nearest "proper" nation to the Eldeen Reaches, Aundair, considers them a terrorist group, and it's easy to see why.
Forgotten Realms 3.5 ed. sourcebookCity Of Splendors - Waterdeep introduced a special feat, "Veil of Cyric". It makes evil characters (but not following an evil deity) undetectable (only) as evil, as they rationalize any acts they "have to" do as just and pure. Cyric is here probably because the Prince of Lies used to see the people who indulged in such self-deception as his personal jesters.
Moving to Planescape, roughly half of the Mercykillers faction can fall into this category; they were originally two separate factions, the Sons of Mercy (Lawful Good) and the Sodkillers (Lawful Evil), but when the Lady of Pain declared that only 15 factions could exist, they joined forces; after the Faction War, when the Lady banished all the factions, the Mercykillers split apart again.
Even worse, the leader of the Mercykillers was like this, and she was one of the biggest allies of the guy who was responsible for causing the Faction War in the first place.
It's also an inherent risk of certain paladin-based prestige classes, such as the Grey Guard (who get cheap atonement when they do something less-than-angelic in the service of good, like beating someone to death to get information out of them) and the Shadowbane Inquisitor (who, despite having levels of rogue, are considered unusually hardassed even by other paladins).
The Monster Manual 3 from the 3.5 version introduces the Lumi, a race of extraplanar light-empowered beings who believe lying to be the ultimate sin. To be clear, if the choice was between telling a lie and letting thousands of people die, a Lumi would have no doubt: all those thousands of innocents would have to die. It seems that these guys are planning a mass invasion of the Material Plane to destroy any and all deceivers...and those who have so much as told a white lie.
In Ravenloft, there's a Darklord who's one of these, a Paladin named Elena Faith-hold who became so extreme (as in, not even her god would support her pogroms against the "unworthy") that the Dark Powers took notice of her and stuffed her into Ravenloft. Every night, she is taken on a ride across her domain to see the spirits of all she has tortured to death, and when she gets back up, she is filled with a desire to make her domain a better place. Unfortunately, this usually means more torture and pogroms. Plus, her "detect evil" actually detects strong passions, so people who like her register the same as people who loathe her, and both usually end up on the chopping block.
The Ravenloft setting is also home to Diamabel, who closely fits this trope, as well as some more sympathetic examples, such as the well-meaning but ignorant Tepestani Inquisition, and monster-hunters or Darkonian Ezrans who've gone overboard in their crusades against supernatural evils.
One of the game's most famous figures tops all these as the game's ur-example and an inversion. According to 3.5 mythos, Asmodeus starts out as an immensely powerful angel tasked with fighting demons and keeping them from wrecking the gods' shiny new Creation. Over time, he proves to be the very best at this task, and he and those like him take on demonic aspects to better fight and kill demons. When mortal species begin sinning and tearing down the orderly lawful framework made to keep the demons out, Asmodeus creates the concept of punishment. The gods love this new idea, and give Asmodeus and his angels the task of tormenting sinners after death, so that living mortals will understand that their actions have consequences and stop misbehaving. Asmodeus and the angels following him take to their new task of torturing the souls of sinners with relish, making the heavens run red with blood and echo with screams. Eventually, horrified by this, the gods sign a contract with Asmodeus to get him and his followers to agree to leave the celestial planes, so they can do their demon-fighting and sinner-torturing elsewhere. They do so, but only to create Hell and begin tempting mortal souls into evil to make themselves more powerful. When the gods see this and charge into Hell to stop it, Asmodeus reveals just how much he's completed his inversion of the trope, pointing out that the contract gives the devilspermissionto do all of this. Or at least, that's his version of what happened.
Asmodeus - "You have granted us the power to harvest souls. To build our Hell and gird our might for the task set before us, we naturally had to find ways to improve our yield."
Hieroneous - "It is your job to punish transgressions, not encourage them!"
Asmodeus smiled, and a venomous moth flew out from between his sharpened teeth
Of course, how much of that legend is actually true is subjective. There is ample evidence that Asmodeus was actually cast into Hell after being banished from Heaven. One thing is true, however: The contract, which is called the Pact Primordial, is very real.
Magic: The Gathering frequently uses this trope with White. In fact, most angels are portrayed as fanatical warmongers. Particular examples include the archangel Radiant (turns a paradise into a police state in the name of her goddess), Akroma, Angel of Wrath (leads a genocidal war to wipe out an evil organization), and Reya Dawnbringer (raises her followers from death, denying them repose).
Akroma's flavor quote fits nicely here. "No rest. No mercy. No matter what."
The Boros Legion from Ravnica. They just want to stop Ravnica's eternal guild warfare. The fact that their method for achieving this is to break huge numbers of heads, blow up a few things, and generally demonstrate the reason guild warfare is bad...is neither here nor there.
"Scores will die in the name of peace. This is what you call compromise?" - Gideon Jura, to Aurelia, Boros Guild Leader, Renounce the Guilds flavour text
Yawgmoth is a Black-mana version: for him, the perfection of Phyrexia justifies any means.
New Phyrexia brought us two different examples: Elesh Norn, the White manaPrinciples Zealot, and Jin-Gitaxias, the Blue mana Totalitarian Utilitarian. Strangely enough, the two of them get along well with each other: Jin-Gitaxias even uses Elesh Norn's sacred book, the Argent Etchings, as part of the basis for his Great Synthesis.
Archons are an entire race of knight templars, representing by default White's more vicious side.
The Knights of the Harrowing from Infernum might fit into this trope, or another trope entirely. They are an order of Christian Crusaders (their old name was the Knights of the Sepulchre) who deliberately decided to transport their fortress into Hell in order to exterminate every last demon, who, by the way, number in the billions...and have a reproductive system where any individual can be sacrificed to produce up to eighteen new demons (which will go from "birth" to "fully grown and ready to kill" in about six months)...and have technology roughly equivalent to the 19th century, backed by Black Magic and demonic innovation (including, but not limited to, machine guns that shoot acid, biomechanical Golems, and rudimentary guided missiles). Also a case of Honor Before Reason verging on Too Dumb to Live.
Peleps Deled from Exalted. Our first introduction has him sparring with a fellow monk over a minor theological point, only to brutally crush her windpipe when he trips her. The question: "Is Terrestrial Exaltation of the Dragons, or from them?" And just to further clarify, this is a man whose actual job is supposed to be hunting the Anathema, and he murders his own colleagues over prepositions. A large portion of his appeal to players and Storytellers is that while most Dragon-Bloods in the Realm can be treated as "good guys" in the right circumstances, Peleps is universally Thunderclap Rush Attack fodder.
Exalted in general who have a high Conviction virtue run the risk of becoming this, especially if it's coupled with a low Compassion.
The point has been made that the detail is important. If the Exaltations are "of" the Dragons rather than "from", it would mean that the Dragons are not in any way responsible for who Exalts as a Dragonblood or why - hence invalidating the Immaculate justification for the Dynasty's "divine right" to rule. (Which is incidentally true, as the Exaltations were intentionally designed to operate completely independent of their creators.) Still, probably not worth killing a fellow Dragon-Blooded monk over.
Ubiquitous in the New World of Darkness. Hunters, Vampires, Werewolves, Mages...all of them have the threat of becoming this hanging over them. (Hunters are "extreme risk," since they're able to modify their code to facilitate hunting... and this tends to send them over the deep end, especially since the average "veteran" hunter, i.e. one who hasn't been dismembered yet, hasn't had a good night's sleep in five years.)
Some Mutants & MastermindsFreedom City villains have these traits. The costumed villain Warden has a particularly extreme case: he worked on making prisons as non-cardboardy as possible, and got a bit fed up with people making that task harder by telling him that the prisoners have rights; didn't they forfeit those when they ended up in prison? His current goal is to overthrow "soft and corrupt" law and replace it with something altogether more draconian.
Iron Legacy, an alternate-universe evil Superman Expy in Sentinels of the Multiverse. He's part of a long line of superheroes, and each eldest child inherits their parents' powers, plus a boost. When his eldest child dies before him, he concludes that he's only got until he dies of old age to end evil forever.
Many factions in BattleTech count. The Clans believe its their destiny to conquer the Inner Sphere and restore the Star League. ComStar still has access to pre fall lost tech, but they think that they should wait out for the Successor States to knock themselves back to the stone age and they would come in an bring enlightenment to the IS.
Heroes in Beast: The Primordial are what happens when you mix this trope with Axe-Crazy and Black and White Insanity. Fixated to a psychotic extent upon the Begotten, Heroes can effortlessly justify any atrocity that they commit by twisting it around and laying the blame squarely on the Beast. Abandoning their wife without any income? Humanity needs them to slay the Beast! Murdering someone who happens to work at the same company as the Beast? Clearly, they were minions of the Evil One! Setting a nightclub on fire and killing dozens of people in the inferno? Collateral damage to try and slay the Beast! It's no wonder that any Hunter who gets to know a Hero either goes jumping off the slippery slope to work alongside them, or, more commonly, tries to kill them for being as big a monster as the Beast they hunt.