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).
Another example: 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.
Baron Klaus Wulfenbach in Girl Genius rules with an iron fist to protect the world from the sociopathic, unwittingly violent genius of the "Sparks". Attempts are made to paint him as a Necessarily Evil ruler whose methods are indeed improving the world, though; it is stated a few times that Klaus doesn't enjoy babysitting Europe.
Klaus is actually an extremely interesting example as he could actually be called a "reluctant tyrant". He hates his job, and would much rather be off adventuring or making crazy inventions (he rarely gets to partake in the latter). As a dictator, his rule basically comes down to "Don't make me come over there!" (Don't start a war, or tamper with forbidden technology of the Other.) Don't break that rule (or make a move to do so), and he is quite content to leave people alone.
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.
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.