"You could not even guess at the things that I have done. Awful, evil, obscene. The telling of them alone could make you puke...They nag at me from time to time, but I tell myself I had good reasons. The years pass, the unimaginable becomes everyday, the hideous becomes tedious, the unbearable becomes routine. I push it all into the dark corners of my mind. And it's incredible; the room back there. Amazing. What one can live with."
Murder, mayhem, and destruction are not things that the average person would readily consider, but if you listen to the average fallen hero, jaded soldier or brutal knight, he'll tell you that it gets easier with time. After that first act of destruction, it becomes easier to stand doing it again, and eventually it becomes second nature. Heck, you may even forget why you braced yourself to do it in the first place.
This trope is often used to segue a character from your average guy to a cold blooded killer; the more he's had to kill, the less he cares each time he's done it. It's also usually part of the backstory for an Hitman with a Heart.
Most stories about The Mafia have this plot line in it, mostly for the main character, who starts out unwilling to kill people, and eventually having no problem killing people when necessary.
This works in Real Life, folks. At least the boot camp has to take a whole year to prepare you to the possibility of killing someone. And they've gotten better at doing it as well, though it must be noted that there are always some people who, despite training, have a mental block that keeps them from pulling the trigger.
Compare Gaining The Will To Kill. Contrast the more idealisticIt Never Gets Any Easier. Closely related to He Who Fights Monsters. Not to be confused with video games that are difficult in the first few levels and get easier later on.
No relation to It Gets Better, which is about a work becoming more engaging after a slow, expository build-up.
Lelouch fits this to a T. He discovers the results of his actions, goes crazy for a bit, does even worse things, and, eventually, has to bluff through his own emotional pain to do the worst/best thing possible.
Suzaku protested about using violence and other extreme measures at first. Not any more.
Specifically referenced by Andrew Waltfeld in Mobile Suit Gundam SEED, and implied to be the case with Mu La Flaga as well, in a deliberate aversion of It Never Gets Any Easier. Waltfeld tells Kira that the first time he had to kill in battle, it made him sick, but after a while he got used to it just as he'd been told he would. And the reason he brought it up was to suggest that WMDs are the same way.
In Fullmetal Alchemist, you see this happen with Bradley in his flashback Cry for the Devil moment. As a young man, he accidentally stabs a friend during fencing practice and is horrified, but is congratulated by his superiors for killing his friend. By the time of the series and following his transformation into a homunculus you have a man who kills without a shred of remorse.
In the 2003 anime version, Ed is upset over having to kill Greed, although he does mention that he accidentally killed minor villain Majahal much earlier in the series. He has no problem with using lethal force against homunculi from this point on, even (or maybe especially) against Sloth, who was created when he tried to bring back his mother, and assumes her likeness.
In Death Note, Light is horrified after realizing that he actually killed his first two victims (which is emphasized more in the manga than the anime), but finds killing everyone else easier after resolving to change the world even if it is extremely painful for him. Eventually, he doesn't care if he has to kill all the people around him as long as it doesn't hinder his goals. It's implied some of this might the effect of the Death Note, but other characters claim it's a natural result of Light's extreme personality being given such a dangerous item.
In Simoun, the Sibyllae are originally very clear that they are priestesses, not soldiers. They are not "fighting," they are "inscribing Ri Maajon." They are not engaged in a "sortie," they are "offering prayers to Tempus Spatium." However, by the time we get halfway through the series, when their country has been at war on multiple fronts for several episodes, they are "on patrol" and "in battle." The newest Sibyllae are fine with that, since they only joined after the war had begun, but for the original members, it is something of a shock once it is pointed out how much things have changed since the beginning.
Fortis of Huckebein from Magical Record Lyrical Nanoha Force tells this to Tohma when he was trying to convince him to join their group of criminals since, as fellow infected, they are his best chance to survive. Touma disagrees.
Fortis: You may resist with the mindset that murder is a crime. However, you'll get used to it. We did too.
In the Warrior Cats graphic novel The Rise of Scourge, it is shown that Scourge, the leader of BloodClan, started innocent but found it easier to kill as time went on.
This is one of the many reasons Batman doesn't kill. If he resorts to the one first kill (he says is often all too easy to fall into it), he may become jaded to humanity and not be able to stop. note This is the modern-day interpretation of the Dark Knight. In the early years of the character — the late 1930s — Batman often killed people and carried a gun.
The Batman example is also an in-universe tactic. In Knightfall, Robin points out that Batman scares the crooks, but doesn't actually hurt them — Batman is quick to point out that he uses it as a psychological weapon, in that the crook thinks he's not worth the effort. Whenever he needs that extra "oomf", Batman always lets drop that there are a lot of unsolved murders in Gotham, so who's to say he doesn't kill...
Checkmate Operative: We have no evidence of Batman ever having killed.
Batman: I fail to see why you think I'd leave any.
In Identity Crisis, a JLA comic, Jean Loring, the Atom's ex-wife, attempts to put the Enlongated Man's wife, Sue Dibny, into fake danger so that all heroes, including her ex-husband, would come closer to their loved ones. After she accidentally kills her, she goes completely nuts and has no problem with putting others in mortal danger. Through this, she indirectly causes two more deaths, and even more indirectly causes the death of Firestorm. (Alternately, Jean is lying about it being an accident; she clearly meant from the beginning to kill Sue (no one "just happens" to be carrying a flamethrower) and one or two other people to cover her tracks. Mind, Jean's still clearly nuts, and her first kill visibly shook her more than the ones she arranged later.)
In Y: The Last Man, 355 gets more and more trigger-happy as the series progresses. And she hates herself for it.
Much of Wanted concerns itself with exactly this trope. It loops around to death being quite bloody horrible once again.
The Invisibles has King Mob, quipping about how after the fifth time, it doesn't feel like murder anymore.
Watchmen: Rorschach is depicted as a Batman-like character who frightens villains rather than killing them, until he crosses the line by slaughtering a child-killer, after which Rorschach routinely kills bad guys justifying as referring to them as "dogs that need to be put down". The film version of the comic also implies this with regards to Silk Spectre II and Nite Owl who are shown slaughtering a group of attackers without blinking an eye.
In Seven Little Killers, Canada becomes a killer. He says that it's like smoking. You hate it at first, but grow to enjoy it. It amounts to him asking America if he can just kill everyone, instead of all of their complicated plans
In True Romance, Virgil the enforcer takes a breather from beating on Alabama to explain his experience with this trope, ending with, "Now I kill 'em just to watch their expressions change." Virgil unforgettably played by James Gandolfini before his big promotion.
This trope is pretty much the source of a lot of internal conflict for the main character in Knockaround Guys.
The last line of the horror film The Strangers is Pin-Up Girl telling Dollface, "It'll be easier next time."
In the Star Wars prequels, Anakin kills dozens of Sand People out of anger, and is consumed by guilt afterward. A few years later, he's hesitant to kill Count Dooku, and eventually does so with reluctance at Palpatine's insistence. Later in the film, he helps kill Mace Windu in a situation of extreme duress, but shrugs it off rather quickly. From there he moves on to watching Palpatine order the deaths of Jedi all over the Galaxy as he himself marches to the Jedi temple to kill everyone inside, including the children. About seventeen years later, Anakin, now badass Sith Lord Darth Vader, has no problem with Grand Moff Tarkin blowing up a planet containing billions of innocent people and is murdering his own subordinates via Force Choke with alarming frequency. A literal case of The Dark Side Will Make You Forget.
Implicity, thanks to his job as top enforcer and commander-in-chief of a galaxy-spanning totalitarian empire, Vader has tons of Offstage Villainy amounting to innumerable atrocities, especially because of this trope and the ease at which he is commiting evil in the movies. The Star Wars Expanded Universe confirmed this, for example having a lethal and highly-contagious biological weapon developed on the Falleen homeworld; and when it escaped and started infecting people in the capital city, the entire area was sealed-off and "sterilsed", ie. annihilated via ion cannon lasers from Star Destroyers from above, killing millions of people.
Dark Blue has a poignant moment after Bobby kills his first suspect. His partner, Eldon, recounts his first kill, and how much it affected him. He says that, even though killing is a normal part of his job now, he still thinks about that first one.
Incorporated to disturbing effect in the Steven Spielberg film Munich. It takes its toll on the characters, however.
In the film adaptation of The Crucible, the first hanging has the girls who had falsely accused them flinching and wincing while the rest of the villagers are cheering. But after the second and third and fourth hangings, they are cheering just as happily as the villagers.
Mr. Pin in The Truth spells this out when his sanity starts getting away from him because he's realized that the people he's killed are closer than he thinks, and are just itching to get their revenge. Killing one person, that's a Moral Event Horizon; killing twenty is just, well, more of the same.
In John Grisham's first novel A Time to Kill, the guy who kills the two guys who raped his kid daughter thinks that it was harder to kill the first Viet Cong fighter.
This is strongly implied to be the case for the murderer in the Lord Peter Wimsey novel Unnatural Death.
Acheron Hades, in The Eyre Affair, describes murder as "like eating a packet of shortbread" — once you start, there's no reason to stop, since the worst they can do is execute you once.
In Tanya Huff's Valor's Choice, Lieutenant Jarret is shaken by having killed someone for the first time, and asks Torin if it ever gets easier. Her response is along the lines of, "yes, sir. I'm sorry to say that it does."
Similar to the Batman example, Hercule Poirot gives this as a justification for bringing killers to justice (after they killed once, they will kill again to avoid being discovered, and each kill will be easier than the previous one). This is an important plot point in his final case.
In Life of Pi, the main character is introduced as devoutly religious, intelligent and a vegetarian. But when he has to survive, he abandons all morals. Killing becomes easier, and soon he is doing things like sucking fluid from fish eyeballs and eating faeces and human flesh. He explicitly states that he goes from crying over a flying fish that flopped into the lifeboat to exulting in the fact that he managed to hook and kill a dorado - and later on, he grabs and slaughters two meerkats without hesitation, so he can rub his feet in their viscera to cool them after he steps on the acidic surface of the island.
In the X-Wing Series, rookie pilot Gavin Darklighter helps out during the Krytos Plague pandemic on Coruscant, trying to find victims to get them treated before it's too late, and call for cleanup teams when it is. In the end stages, the Krytos plague, which was engineered by Imperials, basically liquefies those who contract it. Finding a particularly bad one, someone who'd barricaded himself up when he knew how sick he was getting, makes him vomit, but he pulls himself together, does his job, and confesses to his love interest that a year ago he would have run screaming. He's changing, and it scares him.
Asyr: "It's called maturing, Gavin, and not everyone likes it."
Gavin: "Thanks, but I still have to wonder if it's right that we can see something like that and just continue on."
Asyr: "We continue on, my dear, because we must. [...] Our mission is to fly our X-Wings, to locate and destroy the kind of monsters who would do this kind of thing. Doing that requires all the maturity we can muster."
Repeatedly referenced in Joe Abercrombie's The First Law trilogy. Logen Ninefingers is basically the living embodiment of this trope, which at least still disturbs him.
Enforced in the Sword of Truth series, with the titular weapon. The Sword of Truth inflicts guilt on its wielder every time they kill a person. However, the first time a person kills, the sword inflicts significantly more, to acclimate itself to its new wielder. It's a plot point that being very angry insulates someone somewhat to the sword's guilt, but the Mercy Kill mode of the sword insulates the user completely, because the user is inflicting guilt on himself or herself without the help of the sword.
Ian Fleming inverted the trope regularly in his James Bond novels. Despite the statement made in the 2006 version of Casino Royale, quoted at top, in the original novels and short stories Bond is often depicted as actively avoiding having to kill more than is necessary, leading to some dangerous scenarios for 007, such as in the short story "From A View to a Kill" in which Bond is nearly killed by a man who he shows mercy to.
The Last Guardian by Jeff Grubb has this line used by Medivh after he attacks Khadgar and Garona.
Strongly implied in the first book of The Hunger Games trilogy, wherein Katniss spends much of the first part of the book being concerned about killing people, but once she starts to do so, after a brief moment of self-reflection that mostly focuses on the potential reaction of the family of her victims, she appears to shrug it off and is soon actively planning killing scenarios and, ultimately, putting down Cato without much hesitation, albeit as a mercy kill.
In the third book she kills dozens or possibly hundreds of people indiscriminately, including one scene where she gets in a firefight in a crowded street full of civilians. She has to defend herself with automatic weapons fire and admits that she has no idea what she's shooting at. She almost never sets out to murder people (with two prominent exceptions) but War Is Hell.
This trope is invoked regularly (both straight and inverted) in TV medical dramas, particularly those that feature intern characters. Inevitably an episode will occur in which a patient dies while under the care of an intern and the intern has to deal with the emotions of someone dying under their watch. Complicated in those storylines where the death is due to a mistake made by the intern. In many cases, the emotional dilemma is resolved by having a more experienced physician or nurse explaining that death is part of the job, and they'll either a) get used to it or b) never get used to it (depending on how the storyline goes).
Bones: Brennan is greatly disturbed when she kills for the first time, yet later asks to be given a gun during another case because she's killed before, and in "The Wannabe in the Weeds" she is comfortable enough with killing to shoot a woman in the throat with no remorse evident. (The woman in question HAD just shot Brennan's partner — aiming for Brennan — so hyper-rational Brennan may not have felt it necessary to express or even acknowledge any feelings of remorse, though in the context of the story there's no actual time for this to occur on screen anyway).
Breaking Bad: The first time Walter White kills someone, he is left a sobbing mess. As the series goes on, and as the bodies pile up, he doesn't seem to be as disturbed when he kills, and in season 5, starts to be able to even order the deaths of men with little concern.
I can beat up the demons until the cows come home. And then I can beat up the cows. But I'm not sure I like what it's doing to me. (...) To slay, to kill. It means being hard on the inside.
Cheers: Invoked, of all places, in one episode. Woody, upset that he has told a lie, worries about the consequences.
I've never told a lie before! Wait, that's a lie. It's getting easier! What's next, murder?
Chuck: The trope forms part of the rationale behind the "red test" in which an operative must perform his or her first kill before being promoted to agent. Disturbingly, the kills are of the cold-blooded variety: assassinations and murders, rather than kills in the heat of battle. Sarah and Casey's high body count (and both being willing to kill in cold blood when necessary) attest to the clear implication that it gets easier. The actual episode in which Chuck undertakes his test ("Chuck vs. the Final Test") is built around Sarah being reluctant to allow Chuck to cross the line ( ultimately, Casey secretly makes the kill instead of Chuck; Chuck's actual first kill occurs much later and is not cold-blooded in nature.) By comparison, Covert Affairs, also set within the CIA, does not require its hero, Annie Walker, into a red test and in fact gives her missions despite her not completing firearm training ( she eventually does pull the trigger for the first time).
Doctor Who: In the story The End of Time, we get this exchange.
Wilf: The Master is going to kill you.
The Doctor: Yeah.
Wilf: Then kill him first.
The Doctor: That's how the Master started. It's not like I'm an innocent. I've taken lives. And I got worse, I got clever. Manipulated people into taking their own.
Farscape: Alas, poor John Crichton learned to kill in order to survive the Uncharted Territories. He also went pretty crazy, though whether it was the killing, the utter weirdness, the many, many aliens who decided to stick things in his brain and swirl it around a bit, or some combination thereof is anyone's guess.
Crichton almost certainly had military training, given that he was former space shuttle pilot and they're nearly always drawn from Air Force or Navy pilot ranks. But in the modern era, air-to-air combat is rare and thus few fighter pilots have actually killed anybody. Thus, this trope still holds true over the course of the series.
Fringe: In a second-season episode, after Peter is forced to kill someone for the first time, Olivia recalls her first kill and how it took time for her to get over it. But judging by the rather high body count she's amassed, "It Gets Easier" clearly applies.
In the episode "The Hard Part", Hiro describes his future self in terms of this trope:
"Future Hiro killed so much, he forgot it should be hard."
Sylar goes down that road. After he kills for the first time, he tries to commit suicide (but is stopped by Elle and Noah). We all know how this story continues.
Mash: Had Father Mulcahy getting used to the messy parts of surgery and noted that one thing that pushed him along was seeing the surgeons unconsciously warming their hands in the steam from the opened up torsos of their patients during really cold weather. As he said, "How can you see that and not be changed?"
NCIS: Has also established that in the service's earlier days as "NIS", agents underwent a similar "red test" scenario, carrying out assassinations as a rite of passage. Jenny Shepherd failed her initial assigned kill, though she goes on to commit numerous kills (both hot- and cold-blooded) before the one kill she did not complete years earlier results in her death.
Ziva's somewhat blase attitude toward lethal force in her early appearances on the show, reflecting her Mossad career and the influence of her father, also attest to the trope.
Nikita: The concept is referenced several times in the 2010 version, with the title character, a Hitman with a Heart, driven to take down the organization called Division in part because they made her a cold-blooded killer. Like Chuck, below, Division recruits are also required to complete a cold-blooded kill before being promoted to field agent status.
Person of Interest: Averted Trope in episode "Matsya Nyaya". A woman shoots her boyfriend In the Back, then goes to shoot the protagonist. He says the first time was easier because she's thought about it for a long time and psyched herself up to do it. Sure enough she can't kill a tied-up stranger in cold blood, and flees (only to promptly get shot by someone more ruthless).
Quantum Leap: Reversed the first time Sam Beckett killed a man. The man in question is a former French Resistance fighter who is said to have killed his own mother during the Second World War. After a scuffle, Sam backs away holding a bloodied knife as the man smiles up at him knowingly, whispers "The next time, it will be easier" and dies.
Both Charlie and Aaron have demonstrated this, from being hesitant to use deadly force in the episode "Pilot" to both becoming efficient killers by episode 5 (with Charlie killing ''in cold blood' on at least one occasion (episode 2) and agreeing to do so, though she ultimately doesn't go through with it, on another (episode 6)).
Neville seems to be the end result of the process, going from his pre-blackout life as a man who lacked the courage to confront his Bad Boss and asshole neighbor to a man who not only is willing to slaughter an entire rebel encampment (including noncombatants) but is excited at the prospect of doing so in episode 11.
Comedic example: In The Kids in the Hall, Dave Foley plays a character doing a monologue about being a mass murderer. He says the second kill is easy. "The third time you start to get cocky so you gotta be careful. Y'know, you gotta stay humble or you make dumb mistakes."
The Morality systems in The World of Darkness games are based on the notion that doing bad things to others gradually grows easier (although the specifics are different for each gameline).
GURPS suggests, as an optional rule for "realism", representing this by starting the characters out with the Reluctant Killer disadvantage and then letting them buy it off..
The title character of Macbeth, as indicated by the page quote. Though in that case it's not so much killing as cold-blooded murder (he starts the play as a soldier).
Parodied in The Mikado. The "Lord High Executioner", Ko-Ko, charmed his way into the position, and since no one's been sentenced to death so far, he's essentially just a figurehead. However, when the time comes for him to make his first kill, Ko-Ko protests, "Why, I never even killed a blue-bottle!"
...I'm not ready yet. I don't know how it's done. I'm going to take lessons. I mean to begin with a guinea pig, and work my way through the animal kingdom till I come to a Second Trombone.
Snake's brother Liquid gives a more vitriolic version to Snake in the same game, accusing him of enjoying it. Inverted in Guns of the Patriots; if the player kills an exorbitant amount of enemy soldiers during any one chapter, Snake will have a flashback to the scene with Liquid, and he throws up. For Old Snake, killing gets harder, sort of; more precisely, he gets sick when he realises how easy it's gotten.
One of the major plot points in Metal Gear Solid 4: Guns of the Patriots is the PMC's trying to invoke this trope by using the Sons of the Patriots system to regulate soldiers' emotions. Everything is manipulated so that war literally feels like a video game to them. But when SOP is hijacked, reality comes crashing down, and battle fatigue sets in. What's worse, SOP didn't actually get rid of the emotions: it bottled them up so that when the system was interfered with, they all came rushing to the surface at once. Watching the formerly calm soldiers writhe on the ground, bawling hysterically, or laughing uncontrollably as they beat each other to death with their bare hands, is a very disturbing scene to watch, potentially even more so when you know why.
Inverted in Vampire: The Masquerade: Bloodlines when Jack says, "It's never as sweet as the first time," referring to drinking blood. This is probably a dual reference to drug addiction and the fact that vampires are natural killers. It's also played straight, and used as a gameplay mechanic: killing innocent people causes the player character to lose Humanity points, which makes them more likely to Frenzy and gives them increasingly nasty dialogue options. Jack urges the player to avoid killing innocents whenever possible to avoid sinking too far into their monstrous nature.
In the indie game Vacant Sky, the main character Auria struggles to cope with the knowledge that she's just killed a human being after panicking and killing the husband of their hostess. Though she shows remorse afterward, the subsequent journal entry combined with the fact that the event song is called "A Farewell to Innocence" implies that it only goes downhill from here.
Iji: The titular character starts the game as an Apologetic Attacker and cries things like "I'm sorry!" and "No..." the first few times she kills someone. If you decide to become an instrument of alien genocide, her quotes change to things like "Hah...YOU DIE!" and "AAAARGH!"
In the 2nd chapter of The Spirit Engine, one of your team members stops just short of Heroic BSOD when you have to kill a team of thoroughly Jerk Ass bounty hunters. In the 3rd chapter, s/he's not too happy about having to kill an assassin. In the last one, they're casually slaughtering dozens of soldiers without so much as a sigh. It's similar in the sequel, although not as pronounced.
Outside of the Unfortunate Implications of White Man's Burden, this is the trope the main characters development is based around in Far Cry 3. He starts out as a privileged upper class post grad looking to exploit the jungle paradise with his friends, and ends up getting kidnapped and nearly sold into slavery. In the process of escaping he watches his brother die in his arms, and moments later gains his first kill in self defense, which nearly causes him to break down were it not for the disastrous circumstances of his escape. As the game goes on though, and he ranks up a body count most military platoons would be jealous of, this trope ends up causing him to devolve into a full out Blood Knight. In the good ending, he fully recognizes that he has become a monster due to his kill count, while desperately hoping that there is a way for him to come back from the abyss he's fallen into.
Jak mentions something to this effect to Mizo at the end of Jak X.
In Shadow Complex, protagonist Jason Fleming was trained from a young age by his father to have an easier time when he enlists in the Army...except, much to his father's chagrin, he never enlists, saying that he doesn't want to kill anyone. During the game, he puts the skills his father taught him to good use, as terrorists kidnapping his girlfriend have given him a good reason to kill. Early into the game, he comments that a giant spider-mech is something he can "shoot without feeling guilty," although the driver pops out of the hatch soon after and becomes Jason's next kill. Shortly after that, Jason comments, "Killing's getting easier, not sure if that's a good thing or a bad thing... it's a good thing."
Surprisingly, Carter Blake from Heavy Rain pulls this trope to comfort Jayden if the player made him shoot Nathaniel.
Walker gets a heavy dose of this in Spec Ops: The Line. At the beginning, he is shocked when he has to kill American soldiers. As the game goes on, he looses his reluctance and later becomes bloodthirsty.
It applies to his support team as well. Lugo goes from expressing shock and horror at the viciousness of Walker's actions to casually shooting an unarmed man three times in the head without a second's hesitation, whereas after Lugo gets lynched Adams positively begs Walker to let him fire upon the unarmed civilians who lynched him.
In the first game of the Saints Row series, Troy paraphrases this trope on very rare occasions: "It gets easier every time." If you pay attention to what he's doing whenever he says this (killing a cop), you'll get a hint as to his true motivation for joining the Saints
It comes up again in the 2013 reboot. Lara's first human kill leaves her a traumatized, bloody and sobbing mess. Afterwards Roth tells her it can't have been easy to have killed someone, but she responds that what terrifies her even more is how easy it actually was (granted, it was a life-or-death situation with a heavy subtext of attempted sexual assault or even rape, so...) In-story it eventually does get easier, so that by the time of Roth's death, Lara has come to accept death as part of the reality she's found herself in, and has made a conscious decision to fight back against the Solarii.
Interestingly, Lara's first kill in the game is a deer she hunts for food. She actually apologizes to the animal for killing it afterwards.
Hibiki of The Last Blade 2 is visibly shocked and apologetic the first five or so times she kills (strictly kills - non-lethal wins don't count) an enemy. After the sixth time (as progressively hinted at by her increasingly unhinged win quotes), her win pose changes to one implying she has come to enjoy act.
In Drowtales, Ariel's first kill is forced upon her, but her second is not. She is horrified by how easy it was and develops PTSD from the remorse.
In College Roomies from Hell!!!, when Roger kills a number of Damascus's henchmen, and when Margaret kills Mrs. Pepitone, they're completely dumbstruck, and may or may not have had ill-advised sex. This is the only time any of the henchmen are given a second thought, even by their own side.
In this strip of In Wily's Defense, Megaman refutes this after killing Skull Man.
In The Gamer's Alliance, the thief Refan gradually stops worrying about the people he's killed as he turns more and more into He Who Fights Monsters who succumbs more and more to his demonic side in order to have revenge on those who have wronged him. At one point he comes to realize how far he's fallen but tries to justify it as being the only course of action to take in order to make sure the people who have hurt his loved ones won't hurt anyone ever again...even if it means abandoning his human side and siding with the demon hordes against humans to get the job done properly.
Military experience causes this for many. Though it has been noted that some have no initial difficulty whatsoever while not exhibiting any behaviors indicative of mental illness.
Snipers in the military have a saying "the hardest shot you will ever make is your first".
US Police departments seem to think the opposite, or at least encourage it. Lethal force is without question an absolute last resort and should an officer be forced into that option they are taken off duty for a brief period and receive counseling afterwards (almost) without exception. Most police forces agree that an officer should be able to quickly and efficiently deal with a violent criminal who refuses to be taken in, but that doesn't mean it should be something that's easily brushed off.
Furthermore, in most jurisdictions officers also undergo investigation procedures to determine if a kill was righteous or justified, and face career termination or even imprisonment should it be found the killing was not justified. Although fictional, the treatment of a police sniper in the first episode of Flashpoint reflects what officers often experience, even if they discharge their weapon without injury to anyone. Many police officers have over the years been tried for murder or manslaughter, which could be seen as further discouragement of using deadly force unless necessary. (By comparison, in real life a character such as CSI: Miami's Horatio Caine or Dirty Harry would likely be removed from active duty due to their penchant for lethal force.)
Also, films and TV shows often give the impression that most veteran cops have killed at least once. In reality, if one takes the total number of police officers in the US or Canada one would find only a small percentage have ever had to use lethal force, and an even smaller number have had to do so more than once (unlike, say, Horatio).
Although it is becoming increasingly common for TV and film to lampshade this, mentioning that 95-97% of cops never fire their weapon outside of the shooting range, and that the specific officers we see are the exception. Given that most of these shows deal with one police unit out of cities with tens of thousands of cops, it only starts becoming unbelievable if you're under the impression that all of these shows exist in the same continuity.
Despite the growing quality of simulated-dissection software, medical and nursing students are still expected to engage in human or animal dissections as part of their professional education. This isn't just for hands-on skill development; growing accustomed to grisly experiences and the reality of death is as much a part of these exercises as is building anatomical knowledge. Not a killing variant, but same concept: repeated exposure to death makes dealing with it easier in the future.
The same deal goes for the equivalent animal-oriented professions. Fifty percent of qualified veterinary technicians in the United States wash out of the profession within five years of qualification due to this trope. Technician students are told up front at the start of their training that you either learn to cope, or you quit. There aren't exceptions.
Serious training scenarios for emergency responders (military or civilian) will sometimes use professional makeup artists so the "victims" will have very realistic looking injuries. It's one thing to practice moving someone and pretend they have a broken leg; it's quite another when you see a realistic-looking bone sticking out of what looks like ripped-open flesh from a compound fracture and the victim making an ear-piercing scream if you accidentally touch the wound, combined with litres of realistic blood all over the place.
Ernie Pyle describes this in his wartime column, Brave Men, Brave Men:
The most vivid change is the casual and workshop manner in which they now talk about killing. They have made the psychological transition from the normal belief that taking human life is sinful, over to a new professional outlook where killing is a craft. To them now there is nothing morally wrong about killing. In fact it is an admirable thing.
In a more calm sense, life in general. It seems like a lot of tragedies hit people in their teen years, like breakups, disappointments (not getting into your preferred college for example) and they tend to always be a crisis. In general, later in life, people mellow and develop a sense of "I've been through this once, I can do it again."
Although the TV series Chuck features operatives of the CIA, which exists in real life, the "red test" requirement for becoming a full agent as featured in that series (committing a cold-blooded murder under orders) is unlikely to exist for real CIA agents (at least, as far as the public knows, although Lindsay Moran, in her memoir Blowing My Cover: My Life as a CIA Spy, states that she was never required to kill as an agent). The same goes for the real-life Naval Criminal Investigative Service, which in NCIS the TV series evolved from an earlier organization that used a red test-like scenario.)