Hanging when done properly is a humane as any. Electric chair, injection are just modern pretenses. Electricity stops the heart, so does potassium, then your brain is staved of oxygen and dies.

Guillotine and hanging sever the spinal chord, leaving the brain to die.

If you want to kill in the most humane way, look at the slaughter house. A steel bolt into the cerebral cortex ends the brain activity ASAP. Wouldn't look so good on CNN though.