What is the ruling on someone who marries a woman he knows is unlawful for him, and does the perpetrator become a disbeliever based on the حديث (Hadith) of Al-Bara' ibn 'Azib: "The Messenger of Allah (peace and blessings be upon him) sent me to strike the neck of a man who married his father's wife"?

Scholars have unanimously agreed that marrying a forbidden relative (mahram) is not an act of apostasy but rather one of the grave sins. It becomes an act of disbelief (kufr) and apostasy if one denies its prohibition and believes it to be permissible. The schools of thought differed on the punishment for the perpetrator: The Hanafis believe he should be subject to ta'zir (discretionary punishment). The majority (Malikis, Shafi'is, and Hanbalis) believe he should be given the hadd punishment for zina (fornication/adultery), which is stoning for the muhsan (married person) and lashing for the ghayr muhsan (unmarried person). As for Ibn Hazm, a narration from Ahmad, Al-Khattabi, and Shaykh al-Islam Ibn Taymiyyah, they hold that his punishment is more severe than that of a fornicator and he must be killed in all cases, a killing as a hadd punishment, not for apostasy. They cited as evidence the hadith of Al-Bara' ibn 'Azib, in which the Prophet ordered the killing of anyone who married his father's wife and the confiscation of a fifth of his wealth.