What is the ruling on tearing up a power of attorney left by a father to one of his children, and disposing of it without his knowledge, for fear of affecting the other heirs?

You are not at fault for tearing up the power of attorney, because it has no effect after the death of the principal, unless you intended it to be a will. In that case, the guardian has authority only over minor children. If it is proven that he is unfit, the authorities must be notified to remove him. The inheritance must be divided among all heirs, and each rightful heir must be given their due.