Mishnah 3 in chapter 3 of Maseches Horayos continues and asks the obvious question: what is the law of "chat'u ad shelo nisman'nu" - when the sin was committed while the person was still an ordinary individual, and before he managed to bring his offering he was appointed king or Kohen Gadol? Which offering must he bring: the special offering, or the offering of an ordinary person?
The Mishnah answers: "harei eilu k'hedyot" - they bring the chatas of an ordinary person, for the rule, as we have learned, is that we follow the time at which the obligation was created, and they became obligated to bring a chatas while they were still ordinary individuals.
The View of Rabbi Shimon:
Rabbi Shimon holds that it depends, since in his opinion the time at which the obligation became known also makes a difference:
"im noda lahem ad shelo nisman'nu - chayavin": If not only did he commit the sin, but he also became aware of his obligation to bring a chatas before he was appointed king or Kohen Gadol, he brings a fixed chatas like an ordinary person, since we follow the time of the obligation.
"umishenoda lahem - peturin": If he became aware of the chatas obligation only after the appointment - for example, only after he was appointed king was he informed that the food he had eaten earlier was forbidden - he is entirely exempt. For the Torah defines those obligated in the ordinary chatas as "me'am ha'aretz," that is, ordinary people, excluding a king and excluding a Kohen Gadol; and since he became aware only while he was a king or a Kohen Gadol, he is no longer included among those obligated in the ordinary chatas. He also cannot bring the special chatas, since he did not become obligated in it at a time when he was a king or an anointed Kohen.
The halachah follows Rabbi Shimon.
"V'eizehu hanasi":
Here the Mishnah opens a new topic. The Torah designates the special offering for a "nasi," but the term 'nasi' is ambiguous: sometimes it means the head of a tribe, such as Nachshon ben Aminadav, and sometimes even the head of a family is called a nasi.
The Mishnah says: "zeh hamelech" - it refers specifically to a king, and not to the head of a tribe or the head of a family. "shene'emar: v'asah achas mikol mitzvos Hashem Elokav" - one who does one of the things that Hashem his God forbade him, "asher lo tei'asenah", is the one obligated in the special offering of the chatas of a nasi. From here we learn: "nasi she'ein al gabav ela Hashem Elokav" - this law applies only to one whose leadership is so exalted that no man is appointed over him. Nachshon ben Aminadav, for example, was a nasi and the head of a tribe, but he was subordinate to Moshe Rabbeinu, and he does not stand at the top of the pyramid. Only one who is the king, who stands at the top and is subordinate to no one, was given this unique status of bringing the goat as a chatas.
The Source of the Derivation - A Gezeirah Shavah:
One might ask: how do the words "Hashem Elokav" teach that this refers to one who answers to no one but Hashem his God? The answer is that this is not a derivation from a superfluous expression, but a gezeirah shavah. There is a separate verse that deals with the unique status of the king and with his duty to keep in mind that he is subordinate to Hashem - the verse that commands the king to keep a Sefer Torah with him at all times: "v'haysah imo v'kara vo kol yemei chayav, lema'an yilmad l'yirah es Hashem Elokav". It is precisely one who answers to no one but Hashem, such as a king, who needs this constant reminder. And since it says here "Hashem Elokav" and it says there "Hashem Elokav," we derive a gezeirah shavah - the offering of the nasi too was stated regarding a king.
Practical Halachic Implications:
Two kings in Israel: During the period when there was a king of Yehudah and a king of Israel, since neither king was answerable to the other and neither had anyone appointed over him, both of them would bring the special sin offering of the nasi.
The tribal princes: Since they were subject to an authority above them, they are not included among those eligible for this offering.
It is worth noting that the Gemara raised a question regarding Rabbi Yehudah HaNasi, and Rabbi Chiya answers that he is not among those who bring the sin offering of the nasi - not because of his title 'HaNasi', but because the Gemara says that he was subject to the Exilarch in Bavel, which is surprising in itself, but that is the language of the Gemara. In any event, as a matter of halachic principle, since he was subject to someone else he is not included among those who bring the sin offering of the nasi. Moshe Rabbeinu himself, by contrast, would have brought such an offering, since he was not subject to anyone.