I know of three.
1) When I was a little kid, our neighbor was a 16-year-old boy who would always hide his vodka in our yard. One day dad even told him he had small children, and if any of them got into his vodka he left around, he'd kill him. My dad isn't a very violent man, but he's very protective of his kids! That boy later earned himself 10 years in jail for breaking into a liquor store.
2) I knew this boy in high school who had problems. I was unlucky enough to sit beside him, and once I saw him quietly jacking off under the desk. I claimed I had to go to the bathroom to get away, and thankfully he was done when I got back. He even got a hard-on in school in 10th grade when our teacher was telling us about female circumcision in African countries. He wasn't shy about his erections at all. More than that, he got in trouble twice for sexually harassing girls. One he kept licking his tongue out at in between his fingers to imitate cunnilingus, and another he actually grabbed between the legs. After graduation I heard he was arrested for rape. I have no doubt he probably did it.
3) One girl in 11th grade talked her boyfriend into killing her mother. They're both in jail.