Vaarsuvius's expression upon realizing the Familicide spell killed off Girard Draketooth's entire lineage is the non-verbal epitome of this trope. V has a verbal one in the next strip at the discovery of just how many people Familicide killed due to the way the spell works.
Very early in the strip, Elan pretends to be a wizard and spouted many nonsensical words. Vaarsuvius flips out and harshly insults the bard, causing him to run away crying. Immediately, Vaarsuvius feels awful, and Haley wastes no time chewing the elf out. The two make up in the next strip.
Han: Greedo, old pal! Are you okay?! Greedo? Don't fool around with me, buddy... Oh no!!!
In Girl Genius, it appears that Prince Aaronev Sturmvoraus seems to have had one of these as his daughter is dying, if Tarvek's flashback is accurate. Of course, considering that she is in this condition because he had just tried unsuccessfully to overwrite his daughter's mind with the mind of his Evil Overlord / possible lover, the sympathy one might feel for him is abated some.
Tertaladrol: We're going to be sent to the dung mines of Guanolon IV, aren't we...
During the Great Outdoor Fight arc of Achewood, Ray is attacked by country-western singer Cody Travis, and retaliates by ripping his face off. In the following strip, he looks at his blood-soaked hands and vomits.
In Ask Axe Cop 15 after Axe Cop mistakenly kills a good guy.
Done (to a degree) in response to backlash about Sixx drugging Laura without her knowledge in Collar 6.
Vriska in Homestuck goes through this when she realizes that her actions have resulted in the deaths of most, if not all, of the people she ever considered friends.
Terezi has two examples: once in a subdued manner ( in a dream chat with Aranea over her decision to kill Vriska) and later on says the phrase word for word ( when Gamzee snaps out of his mind control in the midst of her very bloody Big Damn Heroes moment against him and begs her to stop stabbing him).
Riff: Thanks to all the damage you did to the truck, we've lost another day! Instead of being able to free Torg and ZoŽ under the cover of daylight, we'll be lucky to make it in time at all. And that doesn't bother you."
Riff: "Now we'll never make it home in time for the season premier of Baywatch..."
Bun-bun: What have I done?"
Happens in a subtle manner when Torg realizes that he messed up the security computer systems and released a bunch of bad stuff. Torg was inside the No-Fun lab and blew up their server. This causes a bio-weapon mutagen to be released. This may or may not cause turn the entire dimension into a hell like realm full of demon like creatures
Occurs again later when Riff says that future Torg will do bad stuff like cause the Research and Development War or turn this dimension into hell. More like "what will I have done", but still.
Panthera's Jason has developed a tendency to slip into this. To be fair to him, he talked his boss into giving his little sister the same shapeshiftingpowers he has himself... only for things to take a sudden turn for the worst when Ari reveals that he's the very man they have been trying to hunt down and attempts to kill them. The one who kills him off is Onca/Taylor, who utterly freaked out four days earlier at the very idea that her big brother might have killed a person. Ten days after that, they find out that some of the world's leading scientists have no idea how they're still alive and they're going to wither and die in less than thirty years. Cut to the North Korea story arc, where Gyeoknoho comes pretty damn close to finishing his sister off partially because this time, Jason has no idea how he's supposed to come up with a plan to eliminate it. Is it any wonder that Jason is skirting the edge of Heroic BSOD right now?
Part 1 of the Penny Arcade story arc Ripped From Today's Headlines has Tycho feeling bad for accidentally killing his wife.
Anthony Carver in Gunnerkrigg Court reacts in this manner when, in the chapter "Annie and the Fire" describing the events of the chapter "Divine" from his viewpoint. He made a desperate deal to try and bring back the soul of his late wife at the cost of his right hand, but he was ignorant of the connection between mother and daughter. The end result is his realizing, too late and to his horror, that bringing back Surma would come at Antimony's expense. He expresses gratitude that he was stopped before it went too far.
Kimchi Cuddles: Kimchi herself, after hitting her sort-of-metamour Anja.