Hi Pete. Welcome to Websleuths!
Actually someone who murders once and not ever again is the most common type of murderer. More rare are multiples--serial killers, mass murderers or people who murder their whole family.
A deliberate lone killing is often the result of rage, jealousy, an insult or some other perceived injury, and mixed with the perpetrator's use of alcohol or drugs. This type of murder can either be impulsive or planned, though more are of an impulsive nature than premeditated. This category is also represented by accidental killings, unintentional killings and self defense.
Murderers are not all the same, and not everyone who murders has the murdering mindset of a serial killer type. A lone killing could be the result of a person reaching a breaking point.
One of the problems with Roger leaving school is that no one saw him. If we had named eye-witnesses, I'd feel differently about any claimed sighting. I forget the name of the town he lived in. It's currently slipped my mind. It wasn't a large town though, and Roger worked evenings at a local grocery store so that coupled with his skiing abilities should have made him well known in the community, and easily remembered if he'd been seen.
I agree with your third point. If he left school--big if, he'd either have to have gotten a ride with someone or been on foot. If on foot, he couldn't have gotten too far, so if he had committed suicide, I would think he would have been found close to town. The thing with suicide in teens is that most of them carry out their suicides at home. To me, that vitually negates the possibility that Roger committed suicide.
I also agree with your points 4, 5, and 6. I would like to see this case featured on America's Most Wanted. I think what this case needs is a strong dose of publicity.