What do we do with a 16 year old like that? You tell me!
The key is to acknowledge that we do not live in a Perfect World.
That being said, we should not look for a Perfect solution.
Humans are flawed from birth. Including Good Honest people who can turn in an instant, for whatever reason, Your example shows how this is possible. (The Kid changed when he lost his father.)
The same thing happens over and over. Some Good people change to complete opposite when something drastic happens in their life.
A man trying his best to be a Good Human suddenly loses his wife and children to a heinous murder. How can he not help but to be angry at the World----at God---at life!
His mind goes through a physical change. He becomes a sociopath, then a psychopath, then he takes his anger out at other humans because it was a human that killed his family and changed his life.
He starts killing other humans, including innocent ones who haven't experience the reality of this life.
With that in mind, we seek IN-PERFECT solutions to a CURRENT PROBLEM.
Those IN-PERFECT SOLUTIONS are many times DEATH --no so much to punish the man, but to remove the danger to the innocent people that could become his victims.
Is it fair?
NO!
But that's the wold we live in and we have to accept that.
That's why religion was Born. It was humans trying to make sense of it all and give some measure of hope to those innocent oneS who experience the grief of losing the ones you love---FOREVER.