You can execute anyone humanly by using nitrogen asphyxiation.
Place a medical breathing mask over the mouth and nose. Put a head band to monitor brain activity. Apply 100% nitrogen at 1.1 atmospheres to the breathing mask and wait till the brain monitor shows no activity for 5 min.
The person painlessly will pass out in seconds and will be dead shortly after.
Bullet works faster and is cheaper. No waiting to see if he is dead