In a perfect world, Charlie would have done more to talk Mr. Perry out of taking Neil home. He would have seen the signs, called out and made sure Neil would be back, that he wouldn't disappear or go away or die, not without saying goodbye first. And since they are best friends, Neil would have listened – he would have held out (even if it meant switching schools and leaving Welton behind, because at least they could still exchange letters and phonecalls). Charlie knows he would have been miserable, would have hated every waking moment of his father-controlled life from that day forth, but at least he would be alive. It was a selfish notion, he realized, but that didn't stop him from wishing.
In the real world, Charlie thought Neil was safe with his father. He had tried, half-heartedly, to get Mr. Perry to listen – but when it came down to it, he truly thought that no harm could come of Neil, not under the watchful eye of his father. So he had turned his back to the car (half to put it out of mind) and said, "Captain, can we walk home?" He was completely oblivious to the fact that the last expression he would ever see on Neil's face was one of utter anguish.
In a perfect world, Charlie would have stopped him in time. He could have split off from the group and followed the car home, because he sensed something was wrong. He had to make sure Neil was okay because that's what friends do. Not abandon the thought of congratulations and head back to the cave where your friendship had only become stronger. He could have gotten to Neil before anything happened, and everything would be okay.
In the real world, Neil had already made up his mind. For months afterwards, Charlie would hear things said at his parents' parties – "It was the boy's own fault; he would have thanked his father later for putting him through med school;" "So rash and thoughtless! What a shame to have a son like that." – and, suddenly, he wouldn't be able to remember, anymore, if there had been signs. Was it rash? Was it thoughtless revenge aimed at his father, or was it something he'd considered long before he acted? And, really, would he have had any chance to stop it?
In a perfect world, Charlie would have knocked the pistol right out of his hand. He could've crashed through the door and stopped Neil from killing himself, even as his finger was hovering above the trigger. He would make him listen to reason, make him understand that there are reasons to live – if not anything else, Charlie. By killing himself, he would leave Charlie in a world he'd never faced alone.
In the real world, when it happened, Charlie was walking with the rest of the boys back to the school from the cave. It was snowing heavily again by the time they got inside; he shook his cloak off in the front hall and headed towards the boys' rooms. The rest of the guys split, yawning, and headed off to their respective beds. Charlie waited, propped against the wall, outside Todd's room so he could be the first to congratulate Neil come morning.
In a perfect world, Charlie would never have punched Cameron and gotten himself expelled. Cameron's rattiness wouldn't even have been revealed by that point, because the scene would be just like he had set it: Neil would be squeezed into the corner between Charlie and the wall, a book in his lap and glasses on his nose. He would not be dead, and Charlie would have no justifiable reason to punch Cameron in his stupid fucking face, despite the ever-present desire to do just that.
In the real world, he was sent to a military school on the outskirts of Maine. It was a nice place, of course (as anything with a $21,000/year tuition is), but it wasn't the same. He realized not two days into his first semester there that the only reason he had enjoyed his time at Welton had been because of the boys – Knox, Pittsie, Meeks, Todd, Neil. This school had the same characters, but none of them were right. There was something off about them, like it was a picture that wasn't fully colored in. There were geeks and jocks and troublemakers, but there were no geek/jock/troublemaker hybrids, and so he became a loner, the sole person in the entire Academy who just squeaked by in every subject while rowing and reading poetry and wreaking havoc. It wasn't the same, though, not without his partners in crime. He used to do it for the kicks, to amuse his friends – but now that he had no one to care, he just did it because, sometimes, he could hear Neil's laughter, distant, just out of reach.
In a perfect world, he would have told Neil how he felt while he still had the chance. They were friends for almost four years. Not a school day went by that they didn't spend every waking moment together – in any of that time, he could have pulled the boy into an empty classroom and touched the sharp curves of his face in such a way that he would just understand. And there would be no look of disgust because, on the off day that they had been able to get away from school and walk around town, it wasn't only the girls that Neil's eyes followed. And so much time could have been better spent kissing, touching, moaning –
In the real world, he watched Neil fall in love with Todd, and never said a thing.