@tendertouch
Yep. I have a cousin who was very G.I. Joe, rah, rah army, etc... He ended up as a sniper in, I believe, Iraq. He gets very quiet and shakes when he talks about 'taking the shot'. The army trained him to kill, and he did it, but it wasn't easy then and he's still paying for it.
I did some research on this for a character, and most people who kill in immediate self-defense are traumatized by it afterward, some for the rest of their lives.
While I Believe you about your cousin, and His Experiences in Combat, and afterwards.
That is NOT every soldiers' experience.
I recommend you read On Killing by LTC Grossman, who among other accomplishments was an Instructor of Psychology (and other matters) at the USMA WP (US Military Academy at West Point).
I have read extensively books by SLA Marshal, LTC Grossman, and others such as Mates and Muchachos (about combat in the Falklands conflict), since before I enlisted in 1983, and through my career (I retired in 2014). I served as a Cavalry Scout, then transferred to the Infantry, and also served in SOCOM (but NOT as an "Operator" but in as a 'Liaison'). I earned a CIB (Combat Infantryman's Badge), and engaged the enemy.
I am NO 'hero' (but had the Honor of serving alongside several heroic individuals). I fired my weapon, or crew served (and/or) turreted weapons at enemies. At least two occasions I am certain that I was the individual who ended an enemy. They had engaged My Soldiers, and if I hadn't acted their Lives would have been in danger!
Admittedly, I didn't have to view a person through sniper optics! The instances when a kill was confirmed, I wasn't the one to see the body. Nor did I have to photograph them! (We had to take digital pictures because a person we killed might be on a 'Black List' and thus the information was processed by Military Intelligence, and other agencies.
I took photos of others. If it was possible, and it always was for my units. Someone other than the trigger pullers went out to confirm the dead, take pictures, and gather other evidence. This had to do with procedures use of Deadly Force was Always investigated by CID (US Army Criminal Investigation Division) and/or FBI Special Agents; perhaps others too, such as JAG. Also, it was for phycological reasons.
I do occasionally have nightmares, and other issues. But NOT from putting down those who needed it.
There are many factors, and people are individuals. As a Generalization, the Closer you are to a person you kill, the more it is likely to effect you. Killing a person you Didn't intend to can be traumatizing. Viewing a target through a Sniper's Optics is similar to being extremely close; too often you can see their face, their expressions. Often such a target is unaware.
From the 1980's until I retired there were various training programs. One was titled "The Battle Hardened Mind" but It was not so much the program of instruction, but the capabilities of the NCOs who taught us! I was fortunate that in the early and mid 80's there were still Veterans of Vietnam (and other locals) who trained the units I was in. Were my Platoon Sergeant, First Sergeant, and others. Later it was SF personnel, or others who had been in combat (Panama, Desert Storm, the former Yugoslavia), etc. I was in Grenada, and though that was a minor contingency operation, It was a Learning Experience! Being shot at tends to get your attention!
SLA Marshal relates the first hand experiences of soldiers, but the reader has to put some context to it. Men Against Fire, Pork Chop Hill (Korea 1950-53), and Battle At Best (Normandy and the Netherlands in 1944), are among his better books. I also recommend A Soldiers Load and the Mobility of a Nation (it mentions many things such as training, preparation, and nutrition). General Marshal advocated that the effects of Fear and Fatigue are practically identical. That soldiers who are experiencing Fear, become Fatigued more quickly. Soldiers who are fatigued are more prone to Fear (and to Psychological Trauma). That preparing for fear, and Conserving their Soldiers physical (mental and emotional) Energies are among the Most important things a Leader can do.
Learning that others have been shaken, or done foolish things, or acted differently than they were taught, and yet managed to survive, and accomplish their mission is worthwhile.
LTC Grossman is a Psychologist, and read everything written by SLA Marshal and others, and applied scientific rigor to Marshal's theories and speculations; as well as writings of others.