The only proven deterrent is the death penalty. For one thing, it "deters" the original criminal himself, and murderers have been shown by studies to go on to murder again. It could be a guard or another prisoner, or somebody they kill when they get out, but it is obvious that a murderer is like a chicken-killing dog: they just don't stop until you shoot them.
I have also read that in the past in cases where the death penalty was a certainty - murder in the course of a kidnapping or rape, for example - the criminals actually tried to avoid incurring the ultimate penalty. They're dumb, but not too dumb to know what "dead" means.
Of course, now it's no longer a certainty, and if it occurs, probably won't occur until some 20+ years after the crime (unless you live in Texas or Florida, where we are very speedy), so the deterrent effect is somewhat muted. And of course because this kid is a minor, he doesn't even have to worry about it, thanks to the SC and international law.
Well, yes, that "deterrent" is as old as some Freepers (hah!), and I don't disagree.
But what I'm saying is: Your average sociopathic monster is not going to quell his own animalistic behaviors because he realizes someone could put him down. They don't think that way.