It was invented to be a much more humane and egalitarian method of execution than some of the methods that predated it. Nevertheless, its came to be seen as a symbol of the Reign of Terror during the French Revolution.
Like all European countries, France gave special privileges to the nobility not afforded to the common people. One of these privileges was a quick death in cases requiring execution. Whereas the nobility were usually decapitated quickly with a sword or axe, the common people were usually hanged or subject to even crueler forms of capital punishment like burning at the stake or turning on the wheel (a form of punishment that involved multiple breaks to every limb and death by shock and dehydration). This inequality was seen as barbarous in post-Revolutionary France which used the guillotine for all executions regardless of class.
As soon as the National Assembly came to power in France, it ordered a committee to inquire into a better method of execution. This was needed because even the "painless" decapitation sometimes took multiple attempts to kill the victim or killed with blunt trauma that broke the neck. Although the committee agreed that decapitation was the most humane form of capital punishment, they knew they needed to come up with something better than a sword to make it actually painless.
The committee which included Dr. Antoine Louis and Dr. Joespeh-Ignace Guillotin (for whom the guillotine is names) examined the problem and found that similar devices to the guillotine were in use around the world. The Halifax Gibbet the Scottish Maiden and the Italian Mannaia and other devices were examples of machines built to make death swift, if not painless as most of them delivered death by crushing the neck. The committee built from these devices and added a very sharp, very heavy blade that fell from a great height. When released, it decapitated instantly and with virtual certainty.
Instant decapitation did not necessarily mean instant death, however. There were reports that some people lived for a few seconds after being decapitated and may have even been able to blink and see for a brief time. It seems that the quickness with which the neck was severed may have prevent enough damage being done to the brain case to provide instant death. Historians and scientists cannot be certain if the historical accounts can be trusted, but they cannot rule out the possibility that some people survived decapitation for a few seconds.
On March 20, 1792, the guillotine became the official and only legal form of execution in France until capital punishment was banned in 1981. By that time tens of thousands, including King Louis XVI and his wife Marie Antoinette had died on guillotines. It proved so efficient that it was even used in some other countries, notable Germany during the Third Reich.
