Your grandma is not (necessarily - I don't know her, she could be trafficking people) a bad person, but her beliefs and that of so many others who also are good (at least they might be) people provide the fertile ground for the growth of an agressive weed. It's not the grounds fault, it could be growing strawberries instead, but right now its existence nourishes a strangling vine that bears poisonous fruit.
We definetly should not poison the ground to kill the weed, though that certainly is a way to get rid of it. But we absolutely need to prevent it from spreading, new fields should not be infected by it and with the exhaution of the old places of growth, we might manage to extinct it.
That's why it is important to keep in mind that your grandma is (most likely) okay to just exist as a believer, but that the beliefs she holds are roots of something, that must not spread.