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, Superman's enemy Toyman is this way.
Prankster: Why do you do it, "Toyman"? Why do you hurt people? Toyman: Because they deserve it.
Superboy-Prime gradually turns into a Knight Templar throughout Infinite Crisis, as Earth 1 and its heroes were too amoral for him.
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" 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 before being stopped by Batman and Robin.
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.
Judge Dredd himself is one of the best examples of this in the world of comics. In any given strip, there's a chance that Dredd may sentence witnesses or even the victim of a crime after they reported it to him. Notably, in the more emotional story "America", Bennett Beeny gives Dredd a witness statement after he was shot through the throat by democratic terrorists, and immediately after their conversation, the Judges are contemplating whether or not they should arrest Benny for a separate offense. In that one particular instance, Dredd decides to let Beeny off the hook.
Starr from Preacher, a fusion of Templar attitude and Templar position and mission.
Iron Man became one of these during and after Civil War. This need not have happened; both sides were intended to have valid points, but the Editors failed to realise that some of the writers agreed with the Anti-Registration side, so they just kept penning atrocity after atrocity. The main book had him (illegally) clone a god and set him on his former friends, resulting in the death of one. Moreover, the atrocities that didn't involve Iron Man, such as arresting Captain America for refusing to enforce something that isn't a law under the orders of someone who has no authority over him, also took place in the main book. It was a Knight Templar orgy from the beginning.
Spider-Man has often fought a high-tech vigilante called Cardiac who targets people who commit evil and immoral acts, but find legal loopholes to ecape justice. And let's face it; a lot of people would take Cardiac's side here. His victims are horrible men who rob people blind and cause innocents to suffer, but find ways to legally do it, always with selfish goals in mind. Even Spider-Man, who tries to stop him when he can, can't help but admire him a little sometimes.
The X-Men have faced the Purifiers; a sect of Christian fundamentalists led by Reverend William Stryker. The Purifiers believe that mutants are the children of Satan, and they are fighting a holy war against them.
A lot of the mutant-hating human villains, who quite sincerely fear that the super-human mutants seek to replace humanity as the planet's dominant species. (It probably doesn't help that various mutant supervillains seem dead set on proving them right.) Some of them will go to extreme lengths to make the world safe for humanity. One good example is the semi-obscure villain Steven Lang, who will calmly and seriously tell a cute mutant girl to her face that he's sorry, but she has to die.
The Autobots also did this during the Nova Prime administration in the latest series of comics.
Megatron in the IDW comic adaptations of Generation One started out as this. A former gladiator/miner who got pissed at the corruption of the Autobot High Council, he gathered an army of mechs with familiar thoughts toward them and started a typical working-class revolution. However, as the revolution went, he and his army became more and more bloodthirsty and greedy and resorted to more violent methods in the war, and by the time when they finally invaded Earth, none of the original goals of the Decepticons existed. Except for, perhaps, Megatron himself. In one of the most recent comics, he is quoted as saying a rather inspirational passage — "When the word 'Weapon' is emptied of meaning, when the purpose of a weapon is impossible to grasp, when the rejection of my weapon is of significance to no-one other than myself, only then shall I remove it from my arm" — Kinda awesome.
Marshal Law is one, and he knows people think of him as one. And he agrees with those people. As he himself says:
The Decepticon Justice Division are absolutely devoted to the Decepticon cause; they name themselves after the first five conquered cities, and their leader, Tarn, even quotes Megatron's inspirational lines. Their job is to kill Decepticons who are disloyal, cowardly, or disagrees with their goal.
Star Saber is a known religious nut who once proposed destroying all non-believers. With Robot Religion getting a big showing here, fundamentalists think he's mental and kicked him out of their group. Now he works for the final example in the series...
In East Of West this trope is pleasantly avoided by the Ranger who seizes power, kills off the criminal and the corrupted and promptly relinquishes power and retires. Genre Savvy at its best.
In Ultimate Fantastic Four, the Ultimate version of the Psycho Man mind controls a world to feel happy and content, while the Ultimate Silver Surfer argues that they are merely happy slaves. Also, Dr. Doom is always working on creating his "utopia", even if it means destroying the world as we know it. Hey, it's for a good cause.
In the indie graphic novel Artesia, there are the Templars of Agall. These guys worship a patriarchal New God, calling those still dedicated to the matriarchal Old Goddesses heretics. It's their Islik-given duty to protect His church and slay those that oppose them. They have no respect for powerful women like the main character, and in their eyes, any woman who even seems to be dabbling in magic or herbalism deserves to be burned as a witch. Not very nice guys. And they happen to be pretty Badass.
Baron Zemo became one after his so-called reform in Thunderbolts. Zemo crafts elaborate plans to take over the world, but every one is a dressed-up Evil Plan that involves removing free choice from humanity. Furthermore, most of these plans involve Zemo giving himself godlike powers, and he expects everybody to trust him with such power despite his past attempts to take over the world.
Shadow in Archie Comics' Sonic the Hedgehog becomes one in the "X Years Later" storylines. First, once Sonic leaves the timeline, he conquers Mobius and implements a totalitarian regime. He's eventually overthrown and put in stasis by Sonic, but then went From Bad to Worse: five years after being put in stasis, Shadow is freed by his loyalists. Understandably pissed at what happened, he proceeds to release Tikhaos in order to destroy Mobius, so that he can rebuild society afterwards. When Lien-Da protests, he simply kills her, sics Tikhaos on the planet, and teleports away.
Deadlock and the order of the Knights Martial in the ABC Warriors comics are specifically stated to be inspired by the tradition of the Knights Templar.
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.
The eponymous Wanderer from Just a Pilgrim. If you're a raider and you meet up with him, you'd be better off just shooting yourself - there's less talking involved.
Foolkiller from Steve Gerber's Man-Thing believes that he's on Earth to punish fools for being insufficiently godly. At least the first one did. The problem is, a lot of people have held this identity since then, and while all of them targeted "fools" the definition of that term tended to be different for each of them.
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.
Captain Rochnan, the commander of the Vatican's Warrior Monks, in Le Scorpion.
The Punisher, Frank Castle, is one of the deadliest Vigilante Men in the entire Marvel Universe, with a body count that rivals most of Marvel's villains. His watchword is Pay Evil unto Evil, and when he's on one of his many vengeance sprees, the question is not "how far will he go?", but "how fast will he get there?"
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.
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.
Security Chief Parahexavoctal in Buck Godot: Zap Gun for Hire will do anything to enforce his version of keeping the peace, including opening entire embassies to the vacuum of space.