Really killing the 19 y/o would be the most merciful thing for all concerned in my opinion. It not about celebrating the death of the guilty either, there is nothing to be happy about.
I would just hate to see the victims having to relive the horrors in the court room.
The garbage that committed the crime deserves more than the ease of being shot to death. But the trash needs to be disposed of quickly and simply in order to allow the innocent family members the pain of his continued exsistence.
Obviously they are scarred forever the lost limbs are gone, the parents of the boy who died, I suppose something inside them died too, the hurt will never heal.
At least their treasured memories of better times should not be polluted by the unworthy existence of the killers.
I see no value in trying to comprehend the killer just kill him ASAP.