Also, keep in mind, we're not talking about some theoretical possible "someone who might kill." We're talking about actual people who have actually plotted to kill and taken part in it (if not actually killed.) At some point, there is a line that gets crossed, and we can't just say "socio-economic whatever" and have to say: "Sorry, we have to deal with you."
We can address the "socio-economic" thing to those who have not yet crossed that line, perhaps. But as much as I love the idea of peace and pacifism, this is not a problem that is going to be fixed without a willingness to address some manifestations of violence by returning it in kind.