Seriously though? You'd take someone else's life over your own foot or a leg?
I mean...it's grisly, but, it means both of you get to survive all that much longer. And I mean this as a first resort. Full body cannibalism as the last resort. Reason being, humans can survive mmm, maybe a week or two without food, so, a foot for example could keep you for a bit longer. But eventually, without any other food, you'd end up eating your own limbs anyway wouldn't you? So, why not start with the foot? Why kill the other guy first having his carcass rotting and giving you food poisoning? There's quite a bit of meat on a person after all.
Don't start big, start small and work up. Yes, I personally would rather have to get a prosthetic than kill someone the first chance I got...or having myself killed.