Gunnery Sergeant Hartman really cared for private Lawrence. He knew how slow and soft and dim-witted Lawrence was, and that he would in all likelihood get himself and his fellow soldiers killed. Hartman did his best to give Lawrence an environment which would make him tough enough to survive. Hartman even shows pride in Lawrence later on at the rifle range and proclaims that the private is now "Born again hard!" The training just worked too well.
It's really Joker, not Lawrence, who is unfit for service. Joker finds a clearly unbalanced man in the head loading a rifle (a violation of at least three regulations), and makes no attempt to either disarm him (the rifle was empty and leaning against a wall when Joker finds him) or alert the MPs to someone loading a rifle in the head, which results in a DI getting killed and a recruit killing himself. Though given the situation (Pyle's larger size compared to Joker, the rifle leaning against the toilet tank and not accessible to Joker but well within Pyle's reach, Pyle's mental state and potential to go berserk, etc) Joker freezing up and not doing anything isn't necessarily a bad thing.
Why do the two halves of the film seem like two separate movies? Because it shows how different war and boot camp are. They may try to prepare you for what happens in war, but training and trying to shoot people while they try to kill you and your comrades are not the same thing.
By the time Private Lawrence is speaking to his rifle as if it is his only friend, he's pretty much irrevocably insane.
When Hartman is issuing deployment assignments, Lawrence already has the Thousand-Yard Stare. And he hasn't even been in combat yet.
The door gunner is shooting a civilians to simply pass the time.
Crazy Earl's speech on the main page. It's not about winning a war or furthering a great cause. It's a big frat party where you can gun down people and do whatever the fuck you want.
His death counts as Fridge Horror as well — just picking up a strange-looking stuffed animal gets him killed.
Lieutenant "Touchdown's" demise. He was behind a platoon of tanks, surrounded by his squad — as safe as someone could be, one assumes — and he still gets killed out of nowhere.
Doc Jay screaming as he is mortally wounded, over the corpse of the man he tried to save. The squad sees it all and are powerless to help.
Also Animal Muther's blind charge towards them was extremely likely to have gotten him killed. He was just massively lucky — it makes one wonder how much longer he was going to be alive, taking risks like this.
The squad pursuing the sniper was exactly what gets Cowboy killed. If they'd moved on like he said, it would have never happened.
The sniper turns out to be an extremely young Vietnamese girl. This underscores the fact that the VC could be anybody.
This child, who very likely didn't receive any formal army training, nearly takes out a squad of seasoned marines through her incredible aiming skills and her cunning.