My theory is that he had already had murderous tendencies for most of his life, so he jumped at the chance to channel his aggression into legally killing for the Marines. He only realized what a mistake this was when he actually got to camp, and found out how terrible he was at everything military-related that didn't actually involve killing. His skill on the firing range seems to indicate that he had prior experience with guns, so it's definitely possible that he'd tried to prepare for a future shooting spree when he was younger. His murderous breakdown at the end wasn't the result of stress and anger at all—it was just him finally letting go of the aggression that he'd been holding back for years, since he realized that he had nothing left to lose.
- Did he enlist or was he conscripted? And besides, when put under pressure, people break in different ways.
- The Marines, Navy and Air Force have never had a draft. And even then, less than 1000 Marines were drafted in Vietnam or World War II, something which amounted to a recruiter "volunteering" Army draftees to meet quotas in both wars. So he enlisted, he wasn't drafted.
- Many men joined the Marines because they knew they were subject to the draft, and tried to circumvent going to a bad Army unit. The belief was if they are going to fight in Vietnam anyway, may as well be in an all-volunteer outfit. So in essence, many Marines were unwilling, or at least less than enthusiastic volunteers.
- The book the movie is based on is careful to describe Pyle as a very innocent soul. He also isn't out of shape, and is actually described as being skinny. He's just 'slow' and needs to be reminded to do everything. His breakdown is also clearly depicted as a psychotic break: he believed his rifle was talking to him and he shoots the drill sergeant because he looked at 'her' in a way he didn't like. This theory can still make sense within context of the movie though.
- Literally millions of Americans grow up shooting rifles who have no desire to murder anyone. Him already knowing how to shoot (or not knowing and just being naturally talented) isnt indicative of anything. For many people, target shooting is a nonviolent (shooting people is violent, shooting a paper circle isnt) recreational activity. And as I already said, he may have never touched a weapon before and just had a natural talent for marksmanship. That happens sometimes.
Joker seems to really, really want to fit in with Cowboys platoon, despite being a POG reporter who only occasionally sees danger. They seem okay with him at first, and he doesnt cause any problems as long as nothing happens that likely means he might get killed, but the moment things really go sideways, when they take a wrong turn in Hue City, Joker immediately turns into a chattering wreck. His best friend Cowboy has to give him a not-at-all-friendly Joker, shut the fuck up! before his nervous talk starts affecting morale or discipline. Even when hunting the sniper, Joker seems less motivated by any desire to fight the enemy or avenge his friend than by the fact that Animal Mother will probably shoot him if he doesnt. To use Animal Mothers own words, Joker talks the talk, but he doesnt walk the walk at all.
Amongst the gamut of fake veterans are cases of men (and women) who did in fact serve in the military, but claim to have been Special Forces running classified operations when they were actually supply clerks in the safe rear area, or claim to have been career soldiers when they actually were kicked out for disciplinary problems after only a year, or they just claim rank and medals they never earned, and these embellishers often tell the most colorful, war-crime-laden stories about their supposed experiences, with some even publishing bestselling books. Joker clearly wants those around him to think hes something hes not. He will probably carry that with him when he returns to civilian life, spinning Rambo-esque tales of his alleged combat prowess as much to pro-war kids who want to believe in action movies as to anti-war protestors who want to hear about atrocitiesas long as they give him the attention he wants, hell give them what they want.
His logic of everyone being the enemy isn't entirely unfounded as often in the Vietnam war, a seemingly innocent civilian (even women and children) ends up getting the drop on a soldier with some trap or bomb. That perhaps mixed with the "ungrateful" (considering the US was partially helping France with one of their messes) attitude of South Vietnamese caused him to figure "Why take chances? Better them than us".