Death is the worst thing you can inflict on someone. The worst crime - the worst act. It erases a person's entire being.
Now as a society it isn't philosophically or morally right to make a punishment for any crime the greatest crime of all..?
To commit an act this vile on the approval of society makes all members of that society accountable for that act.
When you look at these things in a utilitarian way it becomes apparent that the protection of human life is a concept that transcends justice and law.
An argument for the death penalty is that if the "majority" of a society (or those in power) approve of its use - it is acceptable. Nazi germany held these values.
Anyone who says that this kid got what he deserved is sick. When you live in a society, you should be concerned with its laws - especially if the considerations are on a scale of this magnitude.
The only scenario where the incapacitation/death of this boy is acceptable is if the arresting officers life was severely threatened. - We can't know the exact details of what happened from the media - we may never learn. But I do know that a boy was killed in a police raid - for stealing a computer game console - and the responsible officer has been suspended from duty. The boy may have had a firearm - but how many US homes have guns in them?