Watch what road you go down there, because “preventive execution” as your linchpin rationale could be extended to those who some supposed pundit has deemed likely to murder in the future even if not now. Punishment that is not “deserved” is immoral, period.
You are 100% correct about how wrong it would be to use this science for “preventive execution” of anyone, who has not actually yet committed a crime. OTOH, it would also be 100% wrong to let murderers off easy, on the basis that their brain is defective.
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.