"Private Pyle" was a murderous psychopath all alongAll 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.
Private Joker, scarred by his experience grows up to be a super villainHe 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.