This makes no sense at all.
Whether or not they're alive is kind of irrelevant. You stab them, they go to another plane of existence
Depends on how you think of it. If the Assassin is sneaking in a s trying to do the fight with quick thrust to the heart from behind or poison... sorta useless vs Undead and Demons.
Of course if that is how it worked, it would not explain the combat between the units every time.