These threads are always fascinating, because everyone is so keen to explain it away - it must be a medical condition, it must be because their childhoods were horrible.
It's no secret that my childhood was horrendous, we were assaulted often, my mum was severely mentally ill. My younger sisters were eventually taken into care, my older sister was housed by social services at 15 because she kept physically fighting my mum. I got left, because I was always the one dearming everyone and hiding the knives, rather than being involved in the fights. We begged for social services to do something. Anything. They believed it was better to try and maintain the family status. That my mum might hurt herself if they took us all. At 15, she kicked me out at midnight, after punching me so hard she fractured my skull. I spent the night in a busstop. The council said I could go home or they'd put me in a Salvation Army. They did - and they gave me "Safety training" which involved avoiding the shelter at the hours that people on methadone were at their worst, and barricading the door before I slept.
I'm alright. I've never committed any crimes. I know a lot of similar people, and they haven't either - there's a marked difference between those of us who grew up surrounded by violence and knew it was wrong, and those who just accepted the worldview. The latter tend to be in and out of prison or half-way houses.
Obviously nobody should have a childhood like mine, and some people go through even worse, but it doesn't follow that we'll all become murderers or have no concept of right or wrong. We absolutely should get rid of the current system, it's absolutely not fit for purpose, but we'd still have murderers and cold-blooded killers, and the call would still need to be made on what to do with them.
The Slenderman killings in the US are the same - two 12 year old girls. They had a good upbringing. Probably shouldn't have known about Slenderman at 12, but no indication of abuse or violence or unhappiness.
I don't think you can rehabilitate people without knowing what has made them the way they are. You take the chance to release them again and hope either that they have rehabilitated or that they hated prison enough that they will obey societies rules. If you're wrong, though, it's the rest of society that pays - the family and community of the next victim. I wonder how often that doesn't pay off. But the alternative - to kill them or lock them up for life - seems inhumane.