I recall reading a thread on another site on the the question of how long a decapitated head has consciousness. A French medical doctor present at an execution actually shouted at the executed criminal's head It opened its eyes, and blinked several times before failing to respond. The doctor measure the time as 15 seconds,
So, as to decapitation's being humane, it probably hurts like hell, but for only 15 seconds or so.
Anaestetic before being guillotines? Hang em instead if you got a professional to do it, it is painless or you can't feel a thing because you are uncouncious.