[[WMG: "Private Pyle" was a murderous psychopath all along.]]
All Woobie-ness aside, just look at the guy: he's out of shape, he's forgetful, he's clumsy, he gets scared easily, and he has the worst discipline of anyone in the platoon. Sound like a potential serial killer? No, but it doesn't exactly sound like a potential Marine either. Shooting is literally the ''only'' skill that he has going for him. So why did he even consider enlisting? If he was really so set on serving his country, couldn't he have signed up for the Army or Navy instead? It wouldn't have been a cakewalk, but it wouldn't have been nearly as hellish for him as life in the Marines.\\
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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 UsefulNotes/WorldWarII, something which amounted to a recruiter "[[GotVolunteered volunteering]]" Army draftees to meet quotas in both wars. So he enlisted, he wasn't drafted.
* 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) isn’t indicative of anything. For many people, target shooting is a nonviolent (shooting people is violent, shooting a paper circle isn’t) 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.

[[WMG: Private Joker, scarred by his experiences, grows up to be [[ComicBook/TheJoker a supervillain]].]]
He develops an obsession with smiles after the chilling memory of Pyle's death and now has an obsession with putting smiles on peoples' faces as he kills them. He keeps his nickname from the Marines.

[[WMG: Joker will eventually become a “Stolen Valor” case.]]
While not ''entirely'' a coward, Joker isn’t particularly brave. We see this when he does nothing to stop Pyle before he loads his weapon (which could have prevented two senseless deaths, including Pyle’s), during the first wave of the Tet Offensive, when he nervously stutters “I hope they’re just fucking with us!” and during the sniper scene, where he instantly goes from best friend to TheLoad for Cowboy, who is as scared as anyone but doesn’t let it control him. At the same time, Joker often brags about wanting to see combat, and later about combat he’s been in, as well as the BadassBoast on his helmet. Granted, Joker is clearly a fairly intelligent and educated individual with some rather deep philosophical views on war—though on some level he also seems to think he’s the smartest one around, and he may even be right about that from time to time, though not as often as he thinks—, and his bravado about wanting to be a killer is probably deliberate irony on his part, but he also overcompensates with other Marines.\\
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Joker seems to really, really ''want'' to fit in with Cowboy’s platoon, despite being a “POG” reporter who only occasionally sees danger. They seem okay with him at first, and he doesn’t 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 doesn’t. To use Animal Mother’s own words, Joker talks the talk, but he doesn’t walk the walk at all.\\
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Amongst the gamut of [[PhonyVeteran fake veterans]] are cases of men (and women) who did in fact serve in the military, but claim to have been [[ElitesAreMoreGlamorous 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 he’s something he’s 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 atrocities—as long as they give him the attention he wants, he’ll give them what they want.