Rogue certainly is the most devoted to daemon-slaying amongst the four main characters. While the other three are slackers that could barely be assed to get up off of the couch if they didn't have to, Rogue has been trailed to be a daemon hunter. As such, he views the world through a black-and-white filter, even to things outside of the heretical. This leads him to do a temporary Face–Heel Turn when Rogue threatens to arrest all of Lothar's family, including his father and brother, because Lothar's brother Kyle accidentally sold a shipment of guns to dark cultists. Lothar snaps at Rogue's ultimatum, prompting a fight between the two of them.
Inquisitor Nadia Deket is famous for this amongst the Mobian Inquisition. They even had to change the rules for calling down the Exterminatus because she tried to do it too often. Apparently, the existence of even a vaguely daemonic influence is enough for her to cross the Godzilla Threshold.
Miko Miyazaki a paladin who went from uptight but lawful good to executing the head of her own holy order out of paranoia. Sadly, in her final moments, she starts to realize that what she has done was wrong. Unfortunately, she never gets a chance to redeem herself.
You'd think that when her patron deities, the Twelve Gods themselves, stripped her of her paladin power, she would get the hint that maybe she isn't the hero she thinks she is. She doesn't. Instead, she sees this as just another test to prove her worthiness to the cause... or something.
Redcloak certainly fits this trope. After all, he hates all humans (although, to be fair, they destroyed his home town and killed most of his family). His master plan is to threaten to release a Lovecraftian entity which could destroy the world in order to blackmail the gods into making goblins more equal (in the OotS universe, they were created as Cannon Fodder for PCs to kill), and his contingency plan is to let this happen anyway so the world can be recreated with his god having a say. This version of the goblins' origin was told by an evil god that sponsors Redcloak's plan to blackmail the rest of the gods, and the goblins that do not follow this god, like Redcloak's own brother, Right-Eye, were shown to coexist with humans peacefully. Redcloak almost messed everything up by being greedy, but after spending time with his brother's family, he started to realize that maybe there was another way. That was until Xykon returned and forcibly conscripted the peaceful goblins of Redcloak's brother's village. Sadly, with Xykon's return, Redcloak abandoned the notion of peaceful co-existence for the furtherance of the plan, killing his own brother in the process. This choice constantly haunts Redcloak, even if it is shown subtly. Xykon constantly taunts Redcloak after he loses his eye (a solid reminder of his choice) by calling him Wrong-eye. Redcloak looks in his mirror in his private office and talks to his reflection (the image of his brother), and says, "It'll all be worth it. You'll see." Decades after the event, Redcloak still has doubts.
Raf Maliksh in Dominic Deegan: Oracle for Hire: as the Champion of Law, he took it upon himself to take whatever steps necessary to ensure order — including performing human (well, elf) sacrifices and massacring anyone even associated with the rebellion against him — and when he fell from power, he created The Chosen, a chaos-worshiping cult that he intended to 'purify' the world with. He was so deep in this trope that when he created a personification of Justice, it refused to even listen to him. Most other villains in this comic are this as well.
In It's Walky!, after a particularly painful sequence of events, Sal snaps and goes on a quest to eradicate the evil that the Martians have brought to Earth, which eventually results in her coldly attempting to beat her own brother to death after he attempts to reach out to her, before deciding to wipe out everyone who was ever abducted by the Martians with an alien super-weapon - with the fact that this will also wipe out the entire continent of North America being of little concern to her.
The Gatekeepers in Schlock Mercenary were definitely an example of this trope, cloning and murdering multiple times the entire population of the Milky Way galaxy in order to suppress teraport technology to maintain peace with the immensely powerful Paan'uri before Petey showed them that they've been deceived all along and the Paan'uri were planning to destroy the galaxy anyway.
Kore the dwarven paladin in Goblins, who killed an innocent child because there was a slight chance that he might grow up evil (he was "tainted" from having associated with the so-called "monster" races). The child comes from a genuinely noble dwarven clan and was kidnapped by the monster races. That's right — if you get kidnapped under Kore's watch, your life is over either way. In the past, Kore attempted to claim possession of a powerful axe from a good paladin. In their first encounter, the paladin was deafened; in their second encounter, Kore began by murdering the paladin's wife and children.
Vashiel from Misfile is one of these in his backstory. Apparently, he went too far in punishing a city.
Professor Broadshoulders from Zebra Girl was cursed by a demon in his youth (the curse takes the form of a "yucky face" branded on his forehead), and swore to stop similar things from happening to anyone else. To this end, he tries to kill Sandra, who is a good person despite having been turned into a demon. Recently, it was revealed that the yucky face was actually a demonic third eye, which Broadshoulders opened, damning himself in an attempt to drag Sandra down to hell with him. Ironically, their fight in hell is what pushes Sandra over the edge, from just wanting to be normal to reveling in the infliction of pain.
Miranda West in The Wotch turns out to be an extreme but subtle and secretive case of this.
The Manumitor in City of Reality is willing to kill AV rather than allow her to remain magically transformed into a computer program because of his crusade against transformational magic. Fortunately, he comes around after saving the life of his protege...and being revealed as The Atoner.
Templar in TwoKinds started out as the comic's equivalent of D&D / WarCraft Paladins. Then a high-ranking official went crazy from trying to rez his dead wife and usurped the order, and they now ruthlessly hunt down the Beastmen to "protect humanity".
Seymour from Nosfera fights and kills vampires and other monsters. That's fine and dandy for evil ones like Brahm, but he also goes after good ones too. Rather ruthlessly as well.
In True Villains, the Paladin would surely count as this. He is so dedicated to his fight against evil, he even kidnaps children, and sanctions the use of torture on them.
Othar Tryggvassen, GENTLEMAN ADVENTURER, of Girl Genius is devoted to ridding the world of all Sparks (super-geniuses, for lack of a better term) due to the damage and hurt that some of them bring to the world. He, however, is a Spark himself, and should he ever succeed in killing every other Spark on Earth, he intends to commit suicide. At least he's fair.
Mister Blank of Sam & Fuzzy is willing to go to any length to make Sam the emperor and then kill him when he passes off the title.
White Dark Life has Altair, a veritable poster child for this trope. He blindly wishes to serve God by murdering all that "poses a threat" to Christianity. It's no wonder he and Mysto note herself a Knight Templar who wishes to exterminate the Abrahamic religions to ensure the safety of the Slavic religions (and would have reduced Germany to a smoking crater if she was around while the Nazis were in power; basically, she tries to exterminate anyone who attacks the Slavic peoples) butt heads in the roleplays.
Paranatural: Mention is made of the "Cousinhood of Man," a supernatural organization dedicated to hunting down monsters. Not much is known about them, other than the fact that there are not many of their prey left, and they tend to cause innocent casualties during a hunt. It is the official opinion of the Paranatural Activity Consortium that they are "icky."