How about... when given the item for the limb cutting, I turn and stab the crazy guy? Yes? No?
Okay, on topic...
Probably, if I could get away with just a finger or something, or if there was a very quick way of doing it. No saw-saw-sawing and oh-my-god-I'm-too-feeble-to-cut-through-bone. Because I know I would be. The whole idea makes me twitch but I really wouldn't be able to live with myself if the stranger died. Of course, if after all that it turned out the stranger was some kind of mass murdering lunatic of something... I'd be pretty pissed off. If I knew that from the start I probably wouldn't do it even though I believe that I probably should anyway.