Modified villagers cannot be killed with /kill @e

Modified villagers are given resistance 5 (100% resistance) and instant health. Both resistance and health attributes are set to last a LONG time (we are talking real time weeks here) and CANNOT be killed with even the /kill command. Is this a bug? Or does this mean i have to edit the NBT file to remove them manually? I thought the /kill command was a definite kill no matter the circumstance?