[[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.\\
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 WarTwo, something which amounted to a recruiter "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.
[[WMG: Private Joker, scarred by his experience grows up to be a super villain]]
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.