It read to me as if he was messed up.
All murderers are messed up. They just get messed up for different reasons; this guy sounds like Brevik but without the environmentalism. When I grew up boys were just into boobs and rock music - the worst vandalized shop work tables with dirty drawings of girls and stole bicycles and radios to buy pot, not likely to ever write seething, hateful polemics like this. You couldn’t get them to write a long letter about anything. But then maybe there were really sick ones we just never knew of because they couldn’t find like minds to give them confidence enough to act- there was no social media back then for them to preen on.