Nearly all attacks are reported as "unprovoked" though, purely because they're only reporting one side of the story.
(I'm not suggesting that this particular attack wasn't unprovoked)
It still does in some cases, eg that fella that killed all those prostitutes around Norwich last year.
Prison is supposed to rehabilitate as well as punish. It's not supposed to be society's revenge on the offender.
What good would keeping them in prison for more than 15 years do them? ...or us; the ones paying the bill?
If people were condemned to prison for the rest of their lives, without any discount for good behaviour or confessing or showing repentance, they would have zero incentive to behave themselves inside - it would be chaos and a very dangerous place for Screws and the ODCs.
BTW,
carrying a screwdriver is classed a weapon.