Capital Punishment is not executed due to the convict being a threat. It is done to PUNISH the convict for crimes committed. This argument is as weak as using CP as a deterrent. It ain't called 'Capital Deterrent'.
How about a 30-06 to the head? Think he is strong enough to handle that?
It's got nothing to do with "is he a threat to the citizenry". It is a punishment for the crime committed and to deter others from committing the same heinous crime.