I think the difference is that right-to-die advocates have gone to insane lengths to ensure beyond a reasonable doubt that their setups which administer the lethal substance do so painlessly, so as to ensure that the person willingly choosing to die spends their last few moments not in pain anymore.
Prisons don’t seem to have the same standard in mind with their own setups. If anything, they seem to want to maximize suffering for the sake of the spectacle they’ve arranged the execution around.
A stopped clock is still correct twice a day.