They should use laughing gas.
.
Dentists use it.
Start with a low dose an keep increasing it until it is 100%.
The “patient” would just go to sleep and not wake up again.
Why do we want it to be painless? I think each family should be allowed in a room with the murderer or rapist of their child/mother for one hour. Whatever part of it is left alive should then be hung or shot.
I like straight nitrogen--either by mask or high volume infusion into a chamber. Out before they know it--people have just dropped walking into that sort of environment.