“What I can’t figure out is why he thought he should take his crappy lonely life out on other innocent people.”
He would need to be narcissistic, thinking only about himself. No one else mattered. He was the type who is devoid of empathy. Someone like this would make a bad employee, a bad husband or lover and just a poor companion all around. I would be shocked if he wasn’t also doing drugs. He had probably killed a few animals.
Exactly. Like most killers, he was the center of his whole world. No one else mattered to him. He cared only about his own inner thoughts and feelings: He felt lonely... he never had a girlfriend... him, him, him, everything was about him.
At the same time, he placed importance on other people, as if only other people could make him happy. And, when that didn't happen, he decided to kill people.
Or... maybe his letter was a lie. Maybe he's been fantasizing about mass murder for a long time, and he wrote that letter thinking people would pity him then.
Simple. Everyone else is happy and he wants to make them feel his pain.
After all, “I was surrounded by happier kids who were all going to get what they wanted for Christmas!”