The footnote, as @DoubleAA states, is related to the halachah outlined in the mishna on the preceding Daf. There it is established that keeping that goat would be a sin mi d'rabban, the discussion about the chassid is a continuation of that piece of mishna.
I came across a very intesting answer to your question from Kollel Iyun Hadaf of Yerushalayim, who wrote the following about this specific case on their website dafyomi.co.il
2) THE SIN OF THE CHASID
QUESTION: The Gemara quotes a Beraisa which relates an incident about "a certain Chasid" who became ill, whom the doctors told that
the only thing for him to do was to drink raw milk of a goat every
morning. They brought him a goat, which he tied to the legs of his bed
in order to prevent it from grazing outside so that he not transgress
the prohibition against raising a Behemah Dakah in Eretz Yisrael.
Nevertheless, when his friends saw that he had a goat in his home,
they abstained from visiting him and they called him a robber. The
Beraisa relates that this Chasid had performed no other sin in his
Why did his friends accuse of him of sinning? The Gemara in Sanhedrin
(74a) teaches that when a person's life is in danger he is permitted
to transgress an Isur in order to save his life (except for the three
Isurim of Shefichas Damim, Giluy Arayos, and Avodah Zarah). Hence, he
was permitted to keep the Behemah Dakah in order to save his life.
What sin did he commit?
(a) The TORAS CHAIM in Eruvin (21b, DH Mutav) answers based on the
opinion of the ROSH and the TUR (YD 157) who rule that if a person
wants to conduct himself stringently and allow himself be killed in
order to avoid a transgression, he is permitted to do so. This Chasid
was on such a lofty level of Avodas Hash-m that the act of keeping a
Behemah Dakah in his home, instead of giving up his life, was
considered a sin. (This is also the view of the RAMBAN in Kesuvos
(19a), who writes that it is a "Midas Chasidus" to let oneself be
killed and not transgress an Aveirah. Accordingly, this act was
considered a sin according to the great degree of Chasidus of Rebbi
Yehudah ben Bava.)
(b) The Toras Chaim also explains the Gemara according to the view of
the RAMBAM. The Rambam (Hilchos Yesodei ha'Torah 5:1) rules that one
is not permitted to be stringent and let himself be killed instead of
doing a sin. The Rambam writes that one who lets himself be killed
instead of doing a sin is "Mischayev b'Nafsho" ("guilty for his
life"). Why, then, was the Chasid's act of keeping a Behemah Dakah
called a sin?
The Toras Chaim explains that although the Chasid had no way to cure
himself other than by drinking the milk of a goat each morning,
nevertheless he would not have died without the milk. His condition of
"Gone'ach mi'Libo" would have persisted.
The MAHARSHA (Chidushei Agados) also suggests this answer.
RAV BETZALEL RENSBURG (in Hagahos) questions this approach. The Gemara
in Kesuvos (60a) teaches that a sick person is permitted to drink the
milk of a goat on Shabbos, because even though milking a goat on
Shabbos is an Isur d'Rabanan (Mefarek kel'Achar Yad), nevertheless "in
a situation of pain, the Rabanan did not prohibit it." Here, too, the
Chasid was in pain, and since the prohibition against keeping a
Behemah Dakah is only an Isur d'Rabanan, "in a situation of pain, the
Rabanan did not prohibit it"!
This question may be answered with the words of the ME'IRI. The Me'iri
explains that in a case in which the Isur d'Rabanan involves an
enactment made to prevent loss to other people, one must be
exceedingly stringent. Although the Me'iri himself maintains that the
Chasid should have given his life and not transgressed the Isur
d'Rabanan (since keeping a goat in one's home is a potential source of
damage for others), and the Maharsha maintains that the Chasid's life
was not in danger, nevertheless the Me'iri's explanation that this
Isur d'Rabanan is an especially stringent one suffices to explain the
difference between the case in the Gemara in Shabbos, where the sick
person is allowed to perform the Isur d'Rabanan, and the case here,
where keeping a goat in one's home is considered a sin.
I've included the entire text of the question and answer here, including a link to the website and this article. I hope this falls under fair use.