What is the ruling regarding someone who passed away having bequeathed to disinherit some of their heirs, while ignorant that this is among the gravest sins? And is their will to be executed, or should the inheritance be divided among all the heirs?

A will that disinherits heirs or some of them is religiously invalid and holds no weight. It is forbidden and an overstepping of Allah's bounds, and the testator who makes such a will is sinful. If the testator is alive, they must tear it up, and upon their death, their heirs must divide the wealth according to Allah's prescribed shares. Ownership of inheritance is obligatory and established by religious law upon the death of the inheritee; it does not depend on the will of the inheritee or the heir. Therefore, the estate is divided according to Allah's prescribed shares among all heirs.