Is the person who treated his father brutally considered undutiful to his parents, given that the father instigated most of the problems and disputes with the mother and defamed her and her family?

If a father unjustly slanders the honor of his wife and her family, he is committing a grave sin. However, this does not permit the son to mistreat him, for the father's right is immense, and any mistreatment of his father by the son is a forbidden act of disobedience.

It is incumbent upon the son to enjoin his father to good and forbid him from evil, and to explain to him the impermissibility of slandering honor and oppressing his wife. However, this must be done with politeness and gentleness, without violence, abuse, or harshness in speech, for enjoining and forbidding parents differs from doing so with others.