Imprison him.
Have something stronger to offer as an alternative and offer it. What, do you think prisoners only interact with each other, not with guards, social workers, educators, clergy, visitors of all kinds?
In all honesty, if he is sent to a supermax prison, I would truly consider that a fate worse than death.
Alternative methods of incarceration, to stop prisoners interacting with each other in a ‘Prisoner’s Academy’ have been tried (I can probably find a cite but this is going from memory) in the UK; Lincoln Castle to be exact. When outside of their individual cells the prisoners wore head and neck restraints which meant they could only look forward, they were not allowed to talk and would be given sermons and lessons on a daily basis, to rehabilitate them, in classrooms where all they’d see is the backs of other prisoners’ heads and the guard/teacher/clergy.
A lot of them lost their minds, so many that the practice was discontinued.
I’m all for ‘having something stronger’, I just don’t know what that something is. Whatever it is, it has to act as a deterrent and rehabilitate, or at least stop the damaging philosophies, reasons and/or methods of murder from spreading to others in the prison population. After all, you’re trying to rehabilitate them too. Some prisoners will swap methods for lesser crimes which don’t harm, physically, their victims. I see that as an unfortunate byproduct of a less-than-perfect system.
As to ‘We’re no better’; we have justice systems, rules of law, procedures, precedents, basically an enormous system of rules which have to be followed before a prisoner is put to death. It’s all very calculated and has been developed over centuries. To me, that is better than killing random strangers because you have an axe to grind against not them, but the system they were born into. I’m not saying it’s right, but I don’t agree we’re no better.
All types of society kill people, it is simply a matter of how.
The lib-progressive dream state is killing people right now. Look at the mess in Baltimore and other major cities with the skyrocketing increase in capital crimes. Many are being killed, not overtly by the state, but they are just as dead nonetheless, and their families grieve.
So many can nitpick over what type of a society we are for the DP, but ignore a more massive horror elsewhere.