contrary to popular belief, the guillotine is actually painful for about 15 to 30 seconds as it has been studied by observing the severed heads for the short time they are conscious upon being separated from the body (though this occurrence of being conscious is rare).
Thing is, I don't think people care too much about other people in pain so long as they aren't aware of it.
When your head is quickly chopped off by a guillotine you can't scream (no lungs attached to your airway), and you can't struggle and convulse (besides residual twitching that a body may exhibit, your nervous system has been severed so you can't control your body).
To the outside viewer, there is no pain. Maybe they pull a pained face if they were still alive, but there's no moaning or major signs of distress.
Compare that to a hanging gone wrong, and you can hear someone choke, and you can see them twitch and try to free themselves.
So, maybe there is pain for those who die by guillotine, but it's a short-lived pain anyway, and to the observer, there's not much evidence of it.