Astro City had the Pale Horseman, a vengeful spirit who was manifested by extradimensional energy that was attracted to the populace's Darker and Edgier mood. He targeted all evildoers in his quest, incinerating hardened killers and jaywalkers equally.
During the Knightfall story arc, Batman's fill-in, Azrael, a.k.a. Jean-Paul Valley, was a member of the Order of St. Dumas, a Templar-esque organization of assassins. The second part of Knightfall, Knightquest, tells the story of how Jean-Paul turned from Batman's Legacy Character into a Knight Templar.
Ra's al Ghul was also this, along with his whole League of Assassins. He truly believes that he is purging evil from Gotham, and he's steadily going crazier due to the Lazarus Pit. That's a bad combination!
Another Knight Templar, and a Canon Immigrant, was Lyle Bolton, a.k.a. Lock-Up, who, in the animated series, was once the new Head of Security at Arkham Asylum, but whose methods were so harsh and extreme that everyone at the asylum was afraid of him, particularly Scarecrow. After being relieved of his post, he would go on to "arrest" and imprison those who he deemed to be at the root of Gotham's problems, including the mayor, Commissioner Gordon, reporter Summer Gleeson, and the chief doctor of Arkham — the very same people who exposed his abuse of power and got him fired — before being stopped by Batman and Robin.
Batman himself becomes this in various alternate universes, with one prominent example being Kingdom Come which is set some years in the future and features an older, more cynical Batman who has finally brought peace to Gotham City — by transforming it into a totalitarian police state patrolled by his private army of Giant Mook robots. Some story arcs, such as Batman: Hush, also toy with the idea of the Caped Crusader becoming one of these in the present day, pushing him to some hitherto-unreached breaking point where he considers the idea only to walk him back from the brink via the intervention of one of his trusted confidants (usually Commissioner Gordon).
The Flash: Reverse-Flash II/Zoom/Hunter Zolomon, the Evil Counterpart to Wally West, believes he is improving the abilities of various heroes, especially Wally, by making them experience tragedy. By which we mean he kills everybody who has had so much as a casual conversation with the hero, gloats about said murders, curbstomps the grieving people, and goes all "you'll be a better hero now!"
In the lore of the mythos, the Lanterns were preceded by a robot force known as the Manhunters. A perfect example of the trope, they are the "logical guardian machines removing free will".
Sinestro got kicked out of the Green Lantern Corps for doing this. He had the most peaceful and orderly planet in the universe — because he was ruling it with an iron fist.
After Final Crisis, the Green Lantern Corps create a sort of internal security force called the Alpha Lanterns — using Manhunter technology. This ends up biting them in the ass, eventually.
Kingdom Come. The premise being "what if The DCU experienced a metahuman population explosion, and they became Knight Templars Nineties Anti-Heroes with no regard for collateral damage or civilian casualties, thus forcing the Golden Age and Silver Age heroes out of retirement to set them straight?" Most notable of them is Magog. He ends up repenting though.
Starr from Preacher, a fusion of Templar attitude and Templar position and mission.
Crux from Red Hood and the Outlaws. Obsessed with killing aliens? Check. Willing to go to extreme lengths to do it? Check. Honestly thinks he's the good guy, and people should praise and more over side with what he does? Check. He's so much so that he's literally perplexed when Arsenal attacks him to defend Starfire.
The Spectre sometimes goes this route, especially when he's portrayed as a completely inhuman creature that happens to use a human body as its host. He once considered annihilating New York to avenge the death of a single innocent man. Disproportionate Retribution doesn't even begin to describe The Spectre.
The Elseworld story Superman: Red Son features a Kal-El who lands in Soviet Russia, is brought up as the son of Stalin, and encompasses the world in a prosperous but tightly controlled dictatorship, which deals with dissidents using robotic mind-control on the basis that, hey, it's better thankilling them! In the end, Lex Luthor defeats him by writing him a letter: "I'm distilling everything Superman hates and fears about himself into a single sentence." The contents of the letter: "Why don't you just put the whole WORLD in a BOTTLE, Superman?" Unusual for most Knight Templar characters, this works, and Superman breaks down and cries, realizing that he's no different from Brainiac, who shrunk down cities and put them in bottles — the only thing Superman wasn't able to undo.
In the regular DC Universe, the 90s incarnation of Superman's enemy Toyman is this way.
Prankster: Why do you do it, "Toyman"? Why do you hurt people? Toyman: Because they deserve it.
Interestingly, this Toyman was later revealed/retconned into being an android body double of the real deal, one of many (to explain away his constantly-changing appearance) whose misanthropy was the result of malfunctioning hardware. The real deal is repulsed by his double's Would Hurt a Child tendencies, though tellingly isn't repulsed enough to actually bother destroying it.
Superboy-Prime gradually turns into a Knight Templar throughout Infinite Crisis, as Earth-Prime and its heroes were too amoral for him, albeit one of the decidedly hypocritical variety — in his No-Holds-Barred Beatdown of the Teen Titans, he memorably rants about how amoral and corrupt they are while literally ripping them limb from limb.
The Eradicator, during his early days, was this. When he first gained a humanoid form, he sought to turn Earth into Krypton by force. When he came back following The Death of Superman, he sought to stop criminals his way, by flash frying them.
An alternate Superman becomes this in Injustice: Gods Among Us. He becomes a tyrant and establishes a new world order after The Joker tricks him into killing Lois Lane and nuking Metropolis. Most of the Justice League supports him, except for Batman, who felt that Superman was becoming too extreme. The comics show Superman's slow descent into villainy between the prologue of the video game and 5 years later, when Batman, who has now become the leader of an insurgent group determined to take down the fallen Man of Steel, summons counterparts of the League's members from the main universe to join his insurgency and depose Superman. By the end of the game, Superman and his Regime allies have become not so different from the criminals they fought earlier.
Tales from the Dark Multiverse revisits many of DC's past storylines with dark, twisted versions of its characters... including the aforementioned Azrael and Eradicator.
Its version of Knightfall takes place in a universe where Bruce failed to stop Azrael after he became one of these and thus Gotham spends thirty years as a theocratic Police State ruled by Valley. In the end, a nanite-infused Bruce comes to agree with Azrael and takes over.
The issue dealing with The Death of Superman sees Lois Lane devolve into one of these after she does a Fusion Dance with the Eradicator in her Excessive Mourning, leading her to directly or indirectly kill several villains and even heroes she blamed for the state of the world.
Rorschach. His moral absolutism leads him to continue fighting crime even after superheroics have been outlawed, because evil must be punished, even if that means becoming a vigilante and effective serial killer.
