Zoophobia's Adina believes that she is purifying the world by destroying those she deems "evil", which more-than-often includes those who aren't truly evil.
As shown somewhat earlier in the strip, Miranda West of The Wotch walks full stride into Well Intentioned Extremity in a recent arc, where she punishes Ivan for being too intrusive in his desire to discover Anne's secrets, also condemns two innocents, and then refuses Ivan's plea that he has (apparently) learned his lesson, as well as refusing to help the ones who had nothing to do with her concerns, threatening him when he calls her out on it. But her status as a Well-Intentioned Extremist comes not just from her actions, but also from her attempt to justify it to her familiar (and herself).
Professor Broadshouders in Zebra Girl has made it his life's mission to rid the world of demons - and doesn't care who he has to hurt, damn, or kill to get the job done. Up to and including himself.
BaronWulfenbach, absolute ruler of Europa. He demands obedience of his subjects, kidnaps the children of notable Sparks to his floating citadel in the sky, sends criminals and troublemakers to the CastleHeterodyne to repair it, despite nobody even possibly knowing how to, and has burned entire towns down to the ground to contain unmanageable problems within. He's also absolutely fanatical about keeping the Heterodyne bloodline under control, which puts him at odds with the protagonist Agatha, herself the last known heir of the family. The thing is, he has a point. Prior to his taking control, Europa was a chaotic, feudal place of madness where most Sparks ran wildly and warred against each other with their insane creations, the worst of which included the exceptionally powerful Heterodyne family, which for at least fifteen hundred years had been, with two very notable exceptions, irredeemably evil warmongers singlehandedly capable of bringing Europa into ruin. Everything Baron Wulfenbach has done, however horrific, has been to restore peace and order to the country, and he did a damn good job of it too.Not only that, but once he freezes himself along with the entirety of Mechanicsburg in a bid to keep Agatha from reclaiming the seat of her power, Europa quickly descends once again into a terrible anarchy unmatched by any before.
Redcloak, if one reads Start Of Darkness, falls squarely into this. He wants to improve life for the goblinoid races, but attempts to do so by putting down the other humanoids via divine blackmail, instead of by actually helping the goblins improve their lot.
Even more so, Miko Miyazaki. A case study in what Lawful Good can do, when taken to its extremes. Justice untempered with Mercy, blind to the possibility that she could ever be wrong.
On the heroic side, Vaarsivuus is getting there as well in recent comics. At this rate, (s)he'll end up like Belkar...
However, Faye, Dora, and Marten then quickly point out how wrong this is by asking her what she'd think if a mandid what she did to women who mistreat their boyfriends. She'd ask him out. And then she orders her Vespa-bot to shoot Dora and Faye, with the lasers set to "disfigure".
It's been stated that Roxy Lalonde from Homestuck doesn't really plan her actions often, and they tend to do more harm than good. But she usually means well and enjoys helping her friends, even though she's done things such as blowing up her best friend's computer and coercing her into revealing her feelings for Jake.
A clear example from Homestuck is Aranea, who wishes to create an alternate timeline where the Big Bad never comes to power, and then use her unique abilities to make that timeline become the main universe. Of course, in order to do so, she is willing to kill anyone that gets in the way, including those opposed to the very Big Bad she's trying to stop.
Ian Samael from Errant Story comes from a culture where Elven oppression is common and many Elves wouldn't think twice before wiping him out. His goal to keep his people from being hunted down by Elves is fine; pity that he tries to do so by attempting to wipe out the Elves himself.
Darkbringer from Lightbringer strongly believes that the only way to fight evil is to embrace its ways, abandon all hope, and give up himself to darkness and despair. He believes that Lightbringer's actions give people false hope, so he wants to kill him.
SUEPR Team One, from City of Reality, attempts to protect Reality's Mary Sue Topia by destroying the entirety of the neighboring World of Magic, which has threatened to wreck their way of life. The protagonists are forced to try to stop them, despite nearly falling victim to the very same cynicism.
Weijuaru of Juathuur basically wants to make every juathuur a god, because that would mean absolute freedom.
The robot Blunt in Freefall. Just wants to protect humanity...and has no problems with a plan to lobotomise millions of robots to remove the chance that any of them could harm a human.