Isn't firing squad still considered globally to be the honourable way of execution for soldiers?
A Dutch TV talk show had a forensic pathologist last week to talk about the botched execution in Oklahoma, and they briefly discussed a wide arrange of execution styles - surprisingly, the guillotine was not mentioned, though beheading in general, and Mary Queen of Scots in particular, were mentioned.
As to the question, what the most humane form of execution was, the pathologist first of all remarked that nobody ever lived to tell of their experiences.

But furthermore, he mentioned as part of the cruelty, the angst that comes over the condemned as they walk to the site of the execution, and all the preparations are done.
So, he ventured, that the most humane way would be to tell the condemned they're going to do something nice, so they're unsuspecting, and then give a gunshot to the back of the head.