In the prequel book Start of Darkness, Redcloak goes through a more tragic use of this trope after he kills his brother.
There is a more obscure example in this strip; after Xykon uses a Symbol of Insanity to cause all the paladins to massacre each other, the last one standing recovers and cries, "Oh Twelve Gods... what have I done?" then commits seppukuwhatever Azurites call ritual suicide.
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.
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 Internet 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.
Happens in Sluggy Freelance 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.
In Oglaf, a bunch of villagers approach the Thaumaturge asking her to save them from an evil army of Jerkass goblins. The Thaumaturge claims that evil is relative but jerkassery is not, so she agrees to help them. After the Thaumaturge has incinerated the goblins, the villagers react in a way that makes her realize that they were the true jerkasses all along.
