Even though the guillotine looks like a brutal and painful instrument of death it's creator Joseph-Ignace Guillotin claimed it was a more humane method of execution because the condemned didn't feel a thing as the blade chopped their heads clean off.
And according to Roger Kryson of Quora this claim is true because the blade "immediately severs the nerves from your spinal cord to brain", resulting in but a split second of pain before death:
The clean cut would paralyze you after severing your vertebrae, so pain receptors would no longer send signals as your nerves are severed and your body is non-functional. This is, of course, assuming you’re alive after having your head completely chopped off by a 10-pound blade accelerating at speeds of 40 mph, which you wouldn't be. You wouldn't even feel the cold touch of the blade as it sliced into your neck hair; it would be too fast.
For those saying that spasms have been witnessed after execution by guillotine, it should be noted that spasms such as involuntary jerks, eye fluttering, and twitches can occur up to five minutes after death. This is because the brain suffocates, but it does not mean the presence of pain is there. Many people who pass away naturally and painlessly in a hospital bed will twitch, their eyes flutter, and even have bowel movements minutes after death. Once you are dead, you can't “feel” anything, including pain. As for studies mentioned about brain activity continuing in rats after severing of the head, the same goes. Brain activity can still be present after death, but that does not mean the subject is alive, nor [does it have] the defined senses of feeling.
His answer doesn't take into account the fact that many executioners used to purposely dull the blade to cause their victims more pain, but when it worked the guillotine sliced through necks like a hot knife through butter.
-Via Mental Floss