I should know a way to snap the neck, like a hangman, I guess - but I always laugh at the films that do it so badly...
Not that I know how to do it properly, but I'd have thought it entailed getting behind the victim's neck, which would mean propping the dying guy up, which itself is likely to cause a lot of pain, I'd have thought.
I'm pretty sure commonly acquired poisons aren't painless, anyway, and some of them would be agonising.
it now occurs to me he could be knocked out by a blow to the jaw
Is that actually realistic? I've always thought it a Hollywood thing that isn't actually true. I don't watch boxing, but in real life they get punched quite a bit, don't they? And how many of them are actually knocked completely unconscious as opposed to being a bit dazed thereafter? I know they wear gloves so there's some protection, but nonetheless -- I thought the real damage came from the head rotation, not the blow itself. Isn't the jaw more likely to shatter, causing more pain? And anyone punching the guy could end up with broken hand bones if he's not careful, couldn't he?
I'd go with a sword thrust through the heart. It might not be the most painless, but it's quicker than most and efficient, and they're less likely to bungle it, I'd have thought.
A book I've just read has someone kiss a dying man in the second before he's mercy-killed, to give him something else to concentrate on, I imagine, as well as being a kind gesture. But the kisser-killer is female, so it might not work if your others are great hulking brutes...
NB RJM -- it was Anne. She only required a single stroke, but certainly other victims were hacked about, requiring two or three blows of the axe.