If the lengthy sentence stops just one other teen acting out his or her fantasy, all well and good. 

I'm not saying he shouldn't have had a long sentence but it won't stop others, there is little evidence to show longer sentences for any crime act as a deterrent and in the case of this particular person they weren't bothered by the prospect of prison.

This individual may be at the pinnacle of anti social behaviour which runs right down to the noisy neighbour that doesn't see anything wrong with playing music at full volume at 2.00 am and refuses to stop doing it, it's only a matter of degree.

If we do not attempt to understand why such individuals behave in that manner, we won't find a way of avoiding it and it will just continue.

Unfortunately like with so many other problems, there is little money in preventative research/action, a mentally ill person can tell people or even the police they feel they are going to commit a violent act and no action is or can be taken until it's too late.