Why does the Pasuk begin with the word "Asher"?
Rashi (from Toras Kohanim): It is like"Ashrei" (praiseworthy) - How praiseworthy is the generation whose king has the courage to bring a Korban for having sinned b'Shogeg. How much more will he feel remorse for sinning be'Meizid!
Ramban, Moshav Zekenim, both citing Ibn Ezra: If the one who sinned was the king. 1
Ramban: It is synonymous with "Ka'asher" - 'When'. 2
Rashbam: Invert the words to read " Nasi (ka)asher Yechata" - The Nasi who sins'.
Seforno: It is not a matter of if the king sins, but rather when he sins, as that is what wealth (power) does to a person. 3
Why does the Torah insert "Hashem Elokav"?
Ramban: To remind the king that, although he is the master who need be afraid of nobody, he is obligated to fear Hashem who is Master over the masters. 1
Rosh (1:3): There is no one to demand [that he atone for] his sin. Therefore, no one else offers his Korban (a male goat for a Chatas).
Da'as Zekenim (23, citing Bechor Shor): If only Hashem knows for sure that "v'Ashem", he brings Asham Taluy. When he finds out (the next verse - "Oh Hoda Elav"), he brings a Chatas. Toras Kohanim supports this.
What are the implications of "v'Ashem"?
Ramban, Moshav Zekenim (23): It implies that he sins and is subject to punishments. The next verse adds "Oh Hoda Elav Chataso" - if he brings a Korban and atones, he avoids punishment.
Seforno #1: It implies that he himself is aware that he sinned. 1
Seforno #2 (on Pasuk 27): It implies that he is obligated to do Teshuvah before bringing his Korban. 2
Moshav Zekenim: This implies Mezid. The Torah begins with Shogeg and ends with Mezid, to teach that even if he was Shogeg, he is considered Mezid, for he causes others to sin after him.
Why is Chatas Nasi written right after Chatas ha'Kahal?
Moshav Zekenim This teaches that one who is able to protest, and does not, the Torah considers it as if he sinned.