I knew someone, a family friend, who committed a crime like this because he was mentally ill and his psychiatrists didn't understand how badly off he really was. They sent him home, and the very worst happened... That guy is probably going to be under some kind of supervision forever, but initially he didn't go to prison, but to a mental facility for people who'd committed crimes. I think he was there more than 10 years. I was pretty scared when I heard that my brother was going to reopen contact with our friend, because at first he'd written some long rambling letters to people that certainly made him sound unrepentant. But nothing's happened. They're more or less in touch, the family friend is normal-acting now, he's getting treatment, he hasn't hurt anyone since. It sounds like he's living his destroyed life as well as anyone can after having committed such a crime.
It's really hard to understand what happened to this other kid... but I feel suspicious that he was mentally ill and probably hasn't had mental care during his imprisonment, just because Texas isn't famed for good treatment of prisoners or a nuanced approach to teenage crime So maybe he is actually still dangerous. Still, you'd think he'd be under monitoring for the rest of his life.
I disagree with the evil-kid thing too. For some reason I number a good few special ed teachers among my friends and relatives, and they do have to deal with kids who have emotional disabilities, and are too aggressive or dangerous to be in a classroom. These teachers occasionally talk about a "bad seed" that they've been truly scared by, but such children are, even in their experience of difficult kids, quite rare. I know there's a lot of talk about sociopathy in the media but honestly I think it's a lot easier to train someone into the attributes of evil, through abuse and neglect, than to create a monster fresh.