My linchpin rationale is JUSTICE for the victim. To take a life requires your own life be taken to serve justice.
Prevention of further murders is only necessary or possible if there is already a murder.
The death penalty is for murders not potential murders.
One must be convicted of a capital crime before one can be sentenced to death.
For a life sentence one must also be convicted of a crime.
I certainly do not advocate preventative incarceration.
Well as soon as you lean on “it has a practical purpose of keeping others from getting murdered by the same perpetrator” you have moved your linchpin. Period.