The page image is of Tsar Ivan IV the Terrible holding the body of his son, the Tsarevich (heir apparent) Ivan. The Tsar decided that his son's pregnant wife was too immoderately dressed (or something), and the younger Ivan jumped in to defend her; in a fit of rage, the Tsar proceeded to beat down on his son with his scepter. Heavily. But when Tsar Ivan realized that his son was almost dead, he started kissing him and trying to stop the bleeding, crying, "May I be damned! I've killed my son! I've killed my son!". His son regained consciousness long enough to deliver some Tear Jerker last words, and then remained comatose until dying a few days later—during which his father remained awake for long hours praying constantly for a miracle.
Captain Robert Lewis, co-pilot of the Enola Gay, watched the city of Hiroshima disappear in an atomic blast. Some members of the crew would claim that Lewis was at first caught up in the moment and yelled, "My God, would you look at that sonofabitch go!", but after he calmed down, Lewis wrote in his log, "My God, what have we done?" (The rest of the crew maintained that they did what they had to do).
As he watched the first atomic test ever at the Trinity site:
I remembered the line from the Hindu scripture, the Bhagavad-Gita. Vishnu is trying to persuade the Prince that he should do his duty and to impress him takes on his multi-armed form and says, "Now I am become Death, the destroyer of worlds." I suppose we all thought that, one way or another.
— J. Robert Oppenheimer, Scientific Director, Manhattan Project
Now we are all sons of bitches.
— Kenneth Bainbridge, Trinity Test Director, Manhattan Project
Ashoka the Great, emperor of India in the 3rd century BC, is said to have uttered this line after a bloody military campaign and then converted to Buddhism and never waged war again. That's right, Older Than Feudalism.
Alfred Nobel developed dynamite in order to make the nitroglycerin used for mining, quarrying and construction safer to use. Instead it became widely employed as a weapon, to the point that a paper mistakenly published his obituary on the occasion of his brother Ludvig's death, with the title "The merchant of death is dead". His personal fortune at the time of his death was approximately 250 million dollars in today's money. He bequeathed 0.5% to his family, and the rest to the newly founded Nobel foundation. He also specified that race and nationality was not to be a factor in choosing recipients, which caused a major stir at the time. (The prize committee made good on this specification relatively quickly, giving the Literature Prize to Indian Rabindranath Tagore in 1914.)
Sociologist Robert Putnam was so horrified by the results of a study he performed that among other things suggested that racially diverse societies caused individuals to become more isolated that he almost didn't publish it. He did eventually. When criticized about witholding his findings he explained that he was terrified his work would be used by racist organizations to justify their hatred.
When the hysteria died down following the Salem witch trials, the people of Salem went straight to Psalm 51.
A classic sports example of this happened when Lawrence Taylor gruesomely (but inadvertently) broke Joe Theismann's leg. As soon as it happened, Taylor — known then as a vicious defensive player and a bit of a trash-talker — immediately and frantically called to the sidelines for the medical staff to come and help Theismann, knowing that he'd inflicted an exceptionally serious injury. Indeed, it ended Theismann's career.
Ironically, that didn't stop pundits from thinking LT was making fun of Theismann.
A rather literal example is HMS Beagle captain Robert FitzRoy, who was a fundamentalist Christian creationist. He regretted being part of the Beagle expedition that led to Darwin publishing On The Origin Of Species, thereby indirectly contributing to the development of the theory of evolution, which contradicted The Bible's account of creation. It may have contributed to the depression and anxiety later in life that ultimately led to him committing suicide. The ultimate irony of this being that FitzRoy had taken Darwin onboard as a companion, solely to prevent him from following in the footsteps of his relative Viscount Castlereagh, who had killed himself.
Even more ironically considering how it is viewed now, Darwin considered his work to be proof of God's existence and wrote extensively and approvingly about the religious implications (despite knowing how controversial the work would be). According to his own account it wasn't until many, many years after Beagle that he abandoned his faith. There is a considerable debate as to why he did this, but one possible explanation is the tragic and prolonged death of his own daughter.
A common reaction by people when they kill someone, especially the first time. The degree of trauma depends on a number of factors, such as group participation (squads, teams, etc), range (greater distance = more detachment), emotions and weapons used.
It's a YMMV reaction more than anything else; it is impossible to know how someone will react to killing beforehand. Some people BSOD, others find the experience intoxicating. Most people just feel a bit uncomfortable about it for a while but remain psychologically healthy.
The same holds true for killing animals. Some are Squicked out if they so much as squash a bug, while others go out and kill gigantic animals like elephants and moose for fun.
Stephen King had one of these after he found out about the number of school shootings that had been linked to his novel Rage, in which a high school student snaps after being expelled from school, takes out a pistol, and then kills multiple people as well as hold several students hostage and is portrayed as a Sympathetic Murderer. Investigation of students who had started shootings in their schools revealed that several of them had copies of Rage in their possession. Stephen King then requested that Rage be taken out of print in an attempt to make up for it.
The story goes that General Launcelot Kiggell, Chief of General Staff in the British Army in World War One, saw the conditions at Passchendaele at the end of the battle, cried "My God, did we really send men to fight in that?" and burst into tears. Now considered to be anecdotal, as the higher-ups knew damn well what the conditions were but had legitimate military reasons for continuing. (Whether those reasons constituted justification for continuing is a matter for debate.)
Many raiders who run in on people in their homes threatening the occupants and getting them to tell said raiders, for example, the password to the home safe, will hate having to do this to parents of kids (particularly if the kids are seeing the whole ordeal). There have even been cases in which robbers have turned themselves in due to their guilty conscience about how they traumatized a poor kid haunting them. Some raiders will even try and comfort the kids, if they have to.
Oft experienced after a binge drinking session, especially with some of the more foul tasting beverages.
During the French Revolution, a lot of people felt this way after the execution of Madame DuBary. When she was taken to the guillotine, she screamed, "You're hurting me? But why?" and tried numerous times to escape, only to have been lead back after being caught, or grabbed by another person. Her cries for help and in struggle could be heard from across the city, and it took several men to restrain her as she kept screaming and begging for her life. Even the executioner, who had seen this a lot, was getting unnerved by how much of a struggle she was putting up, and instead of crying for blood, several people in the crowd were walking away, really unnerved, and some were allegedly crying for her to be spared. Needless to say, a lot of people felt this.