I can think of no other appropriate penalty for the perpetrators of this act - certainly, with particular regard paid to the cold-blooded vermin who pulled the trigger and arbitrarily removed a life and damaged other members of the victim's family for the remainder of their lives.
It is 100% morally unacceptable that our society grants people the right to slay others in the full freedom and knowledge that their own life will never be under any threat as a consequence.