Marriage to a mahram woman is a major sin but does not mean that one is an apostate

As far as we are aware, the scholars of the four madhhabs and
others are unanimously agreed that marriage to a mahram or to someone to
whom marriage is not permissible does not constitute apostasy in and of
itself, and it does not constitute disbelief that makes it permissible to
fight a person and seize his wealth. Rather it is one of the major sins and
shameful acts of disobedience to Allah. It does constitute disbelief and
apostasy if that person denies that it is prohibited and believes that this
marriage is permissible. 

The scholars differed concerning the punishment of the one
who does that, but none of them suggested that he is to be executed for
apostasy. 

According to the Hanafis, there is no hadd punishment for
that; rather he is to be subjected to a disciplinary punishment (ta‘zeer).
They interpreted the hadith mentioned by the questioner as referring to one
who regarded his action as permissible.

Ibn al-Humaam al-Hanafi (may Allah have mercy on him) said:

This indicates that he regarded that as permissible, and thus
became an apostate. End quote from Fath al-Qadeer (5/261)

The majority of the Maalikis, Shaafa‘is and Hanbalis are of
the view that anyone who marries a woman who is his mahram should be
subjected to the hadd punishment for zina; if he was previously married then
he is to be stoned and if he was not previously married, then he is to be
given one hundred lashes. They also interpreted the hadith of al-Bara’ ibn
‘Aazib as referring to one who regarded this deed as permissible. 

It says in at-Taaj wa’l-Ikleel, which is a Maaliki
book (8/370): 

If he marries the wife of his father or the wife of his son,
he is to be subjected to the hadd punishment [for zina], if he was aware of
the prohibition on that. End quote.

The Shaafa‘i scholar al-Maawardi (may Allah have mercy on
him) said:

Because he regarded as permissible that which Allah, may He
be exalted, had definitively stated was prohibited, the Prophet (blessings
and peace of Allah be upon him) regarded him as an apostate, and by taking
the khums he classified his wealth as fay’ (a kind of booty).

End quote from al-Haawi al-Kabeer (8/146). 

The Hanbali scholar al-Bahooti (may Allah have mercy on him)
said:

With regard to the report of al-Bara’ (about executing a man
and seizing his wealth), Abu Bakr said: According to Ahmad, it is to be
understood as referring to one who regarded that prohibited act as
permissible, and that the one who does not regard it as permissible is to be
treated as a zaani (fornicator or adulterer). 

End quote from Kashshaaf al-Qinaa‘ (6/54-55). See
also: al-Furoo‘ by Ibn Muflih (10/56). 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him)
said:

With regard to the hadith of Burdah ibn Nayyaar, according to
which the Prophet (blessings and peace of Allah be upon him) sent him to
someone who married his father’s wife, and he instructed him to strike his
neck and seize one fifth (khums) of his wealth, the fact that he seized one
fifth of his wealth indicated that he was a disbeliever, not just an
evildoer. He was deemed to be a disbeliever because he did not regard as
prohibited that which Allah and His Messenger had prohibited. 

End quote from Majmoo‘ al-Fataawa (20/90) 

What Shaykh al-Islam (may Allah have mercy on him) meant when
he said that “he did not regard as prohibited that which Allah and His
Messenger had prohibited” is that he regarded that act as permissible, as is
indicated by the words before and after that. 

Shaykh al-Islam (may Allah have mercy on him) was asked about
a man who bought a slave woman and had intercourse with her, then he gave
her to his son. Is it permissible for his son to have intercourse with her? 

He replied: 

It is not permissible for the son to have
intercourse with her after his father did so. This is according to the
consensus of the Muslims. Whoever thinks that that is permissible should be
asked to repent; if he repents, all well and good, otherwise he is to be
executed. In as-Sunan it is narrated that al-Bara’ ibn ‘Aazib said: I
saw my maternal uncle Abu Burdah carrying his banner and I said: Where are
you going? He said: The Messenger of Allah (blessings and peace of Allah be
upon him) has sent me to a man who married his father’s wife, and he has
instructed me to strike his neck and take one fifth of his wealth. There is
no disagreement among the leading scholars that it makes no difference
whether he had intercourse with her on the basis of marriage or on the basis
that she was his slave. 

End quote from Majmoo‘ al-Fataawa (32/77) 

Ash-Shawkaani (may Allah have mercy on him) said: 

It is essential to interpret this hadith as meaning that this
man, who the Prophet (blessings and peace of Allah be upon him) issued
instructions was to be executed, was aware of the prohibition on that deed,
and he did it on the basis that he regarded it as permissible. This is
something that implies disbelief, and the apostate is to be executed. 

End quote from Nayl al-Awtaar (7/137) 

Ibn Hazm was of the view, which is also narrated from Ahmad,
that the punishment of one who has intercourse with a mahram should be more
severe than the punishment of one who commits zina. He must be executed in
any case, whether he was previously married or not. 

However they said that he is to be executed as a hadd
punishment, not because of apostasy. 

This view was favoured by al-Khattaabi and Shaykh al-Islam
Ibn Taymiyah. 

Ibn Hazm (may Allah have mercy on him) said: 

Whoever has intercourse with his father’s wife on the basis
of a contract that he calls marriage, or without such a contract, must
inevitably be executed, and seizing one fifth of his wealth is obligatory.
The rest of his wealth goes to his heirs – if he was not an apostate – or it
goes to the Muslims if he was an apostate. 

End quote from al-Muhalla (12/204) 

Ibn al-Qayyim (may Allah have mercy on him) said: 

By Allah, (the Prophet (blessings and peace of Allah be upon
him)) did not merely rule that he was to be subjected to the hadd punishment
of zina; rather he ruled that his neck was to be struck and his wealth
seized. And this is the pure truth, for his crime was greater than that of
one who commits zina with his father’s wife without a marriage contract, for
such a one commits one sin, whereas the one who does the marriage contract
with her adds to the sin of intercourse the sin of a marriage contract that
Allah has forbidden. Thus he transgresses the sacred limits of divine law by
doing such a marriage contract and he transgresses the sanctity of his
mother by committing incest. 

End quote from I‘laam al-Muwaqqi‘een (2/249). See
also: Zaad al-Ma‘aad (5/13-14); al-Mughni (9/56); Majmoo‘
al-Fataawa by Shaykh al-Islam Ibn Taymiyah (34/177); Ma‘aalim as-Sunan
(3/329) 

Conclusion: 

Although this deed is emphatically prohibited and is a
serious crime, it does not constitute disbelief in and of itself, as is
indicated by the words of the scholars concerning the hadith quoted above,
unless the person who does it regards it as permissible, and does not submit
to the ruling of Allah and His Messenger concerning it. 

And Allah knows best.