Alright, I hate doing this, because it makes me sound like a major douche, but there is a Major flaw in this RP idea.

You can't really run a "Would you sacrifice your morals to survive?" RP in medieval times, primarily because daily life was already like that, with little food, most medicine being detrimental to your health and crime more rampant than you would believe. One of the main themes of a Zombie Apocalypse setting is that your characters tend to be forced out of their comfort zone. From "Safe and secure at home" to "Everybody in the world wants to eat your brains." It's learning how to survive without modern equipment. You adapt or die. But, again, that was pretty much already the case.

Essentially, the middle ages were essentially the Walking Dead, but without the guns and Zombies.