I wonder if any pets have "mysteriously disappeared"... If he was really set on killing someone, which it looks like he really was, then it was only a matter of time.
Even as adults we say that. We don't actually mean it and we don't actually kill people. When I was in 9th grade, one of my classmates was in a really strange situation. His girlfriend, who was in 8th grade at the time, went to his house and told him that if he didn't ran away with her she would kill him and then herself. She had a knife and everything. No game plan though further than that. My classmate went with her but they didn't go very far, they stayed in the woods for a few hours until she realized she had no other option than going back home.
Very strange. The girl was quiet and I never thought she was violent or suicidal. I have no idea why she did it. They kept dating even after that incident. Our minds are a little weird. But all of this to say that, if that girl ever told me she was going to kill someone BEFORE that happened, I wouldn't have thought about it, but if she told me that AFTER that incident I would've 100% told an adult about it.
I can't really blame the parents either, I'm an adult now but I remember being 13 and 14 and being sneaky. If I wanted to lie, I would lie and my parents never knew. It's hard to control your kids when you're working, life keeps going and no parent can't control their kids 24/7, especially at 13, 14.