That case had little if anything to do with the right to life. It had everything to do with deciding who speaks for a person when that person is unable to speak for herself.
The power of life and death apparently resides in the State. For an individual citizen to attempt to assert such power without sanction of the State is asking for the State to come down hard on that individual citizen. This extends not only to criminals, where the power has been obvious for hundreds of years, but to any citizen. And even non-citizens who interact with the State on their own such as captured enemy combatants [you will find this power mentioned under piracy on the High Seas during the Constitutional Convention of 1787].