It is rather curious that we consider the guillotine cruel. It is a much faster and certain mode of execution than a firing squad, lethal injection, or hanging. Which actually makes the guillotine more humane than those other methods.

The problem with the guillotine is gore, not cruelty. It is bloody and messy and it makes it really, really clear that you are killing a human being. We prefer to think that we are "terminating criminals" or performing some other act of impersonal justice or retribution, rather than simply perpetuating murder.