What would happen if the death penalty was optional for anyone of sound mind facing an extended prison sentence?

Anyone judged to be sane that receives a sentence that effectively ends their life.

A 26 year old man facing 45 years in prison.
A 70 year old pedophile facing 20 years.
Anyone facing life without parole.

They could simply ‘call it’ and choose to be put to death in a humane way.

How would it change society?