I like your point. I think another poster said the same thing. He may have convinced himself it was completely justified and self-defense. Like the OJ defense, if I believe the lie long enough, it's not a lie...it's my truth. This is why a deathbed confession is not coming. This is why he wouldn't worry about it for 45 years...what's to worry about? He didn't do anything wrong- in his twisted mind
Along those same lines, the person who did this had some severe psychological problems to kill Henry in the first place. Those psych/mental issues were in operation before and after the murder and allowed the killer to feel no guilt about what he did. No conscience. Maybe he was used to not being held accountable to others.
For that reason, JMO, it's likely this killer stood out to other students in school and around town. Maybe not in a big way, but I'd be willing to bet he was someone known to have a temper or to bully others. If he came from a "good" family, his behavior may have been overlooked or dismissed by others.
Another possibility is that he was considered an oddball or loner, someone whose violent tendencies were not obvious due to social isolation.
Regardless, he was a schemer. He was someone who took the time to arrange a place and time for his meeting with Henry and bring along a weapon.
He arranged it in a way that didn't arouse suspicion from Henry.
He was organized enough to escape unnoticed and dispose of evidence (bloody clothes, etc.) before cleaning up and returning to "normal", not an easy task for a minor who still lived at home with his parents. Are we looking for someone whose parents worked and weren't usually there when he got home from school?
Henry's murder doesn't appear to have been an impulsive act, it was planned by someone who could trick Henry into going to that spot that busy afternoon.
Just a hunch, but I suspect some friends and fellow students of Henry's had or have suspicions about the killer. For various reasons, though, they rationalized that it couldn't be him.
JMO