The "killer" in Pvt. Joker only came out when the Vietnamese sniper was begging to be put out of her misery. Not even in retaliation for the damage she caused to his platoon. And the killing was an internal fight against all of Joker's peace-loving instincts.
Pvt. Pyle was systematically tormented by Gunnery Sergeant Hartman, and finally by his platoon, to the point of insanity and need for revenge for that torment, at least against the Gunnery Sergeant. Before basic training Pyle was just a simple slow and slobbish guy, with no apparent latent violent tendencies.
So, "PvtJoker", you couldn't reply to my previous "detective first grade" connection between your handle and the movie? I'm a little disappointed...lol