I would like to say, "Yes, of course!" But that would just be a huge lie.
Chances are, we are both going to die anyway. Do you really think that crazy guy is just going to let you go after he kills the stranger? Most likely, "Noooooo.". He'd probably kill you for being so selfish - if he were anything like the Saw villain.
I probably wouldn't even cut off my own limb to take a stab at saving myself. The chance of actually surviving long enough to get help with a severely bleeding stump, is minimal. Depending on where he's keeping us , and the tools at my disposal.
If the limb was properly, medically amputated, by a doctor and I would be under, then sure; I'd save that other persons life and live with a prosthetic.
If I have to hack and slash off my own limb in a dirty alley or dirty old bathroom with a saw,...no chance. Noooooo chance. lol