Why does the Torah write "Nefesh" in connection with the Korban Chatas?
Ramban, Moshav Zekenim #1: Because the Chatas comes for a sin that is performed b'Shogeg, which is a matter of Machshavah, which in turn, is a function of the Nefesh.
Ramban #2, Moshav Zekenim #2: Because every sin creates a blemish in a person's Nefesh, preventing it from coming before Hashem until it has been atoned for by means of a Korban Chatas.
Ramban #3 (citing Toras Kohanim): it comes to include Geirim and Avadim in the realm of Korban Chatas.
Moshav Zekenim 1 #3 (from Vayikra Rabah): Hashem judges primary the Neshamah for sin, for it comes from a place of Taharah, and sins and does not remember its source - 'Nefesh ha'Chota'as Hi Tamus.' The body comes from a place of Tum'ah; it is not punished so much. For Mezid, both are punished together. This is like a blind man and a lame man who were told to guard a field, and they ate the fruits
Da'as Zekenim bring similar to this. He says that in the future, Hashem will judge the body and Neshamah together.
What does "mi'Kol Mitzvos Hashem" (in connection with a Korban Chatas) incorporate?
Rashi: It incorporates Lavin 1 that are subject to Kares and that one transgresses b'Shogeg.
What can we learn from the 'Mem' in "me'Achas me'Heinah"?
Rashi and Rashbam: One is liable for a partial Aveirah, e.g. someone who writes on Shabbos part of a word 1 - such as 'Shem' from Shimon, 'No'ach' from Nachor' or 'Dan' from Daniel is Chayav to bring a Korban.
Two letters that themselves make up a word.
Why does the Torah add "asher Lo Se'asenah"?
Rashi (in Eruvin, 96a) and Rashbam: To preclude someone who transgresses Pesach or Milah b'Shogeg from bringing a Korban Chatas, which are Mitzvos Asei. 1
Even though they would be subject to Kares if they transgressed be'Meizid.
What is the reason for Korban Chatas?
Rosh (1:3), Da'as Zekenim (2:13): Hashem creates the cure before our wounds. When a person sins, and offers a Korban and atones, and knows that he is clean from his sin, he is careful not to dirty himself again. If not, once a person sins and repeats it, he considers it like Heter. One who wears dirty clothes does not guard them from mud. "B'Chol Es Yihyu Vegadecha Levanim" (Koheles 9:8). 'Hashem wanted Lezachos (to cleanse) Yisrael, therefore He gave to them much Torah and Mitzvos', to engage in them and not get dirty.
Sha'arei Teshuvah (4:2): An Aveirah for which there is Kares b'Mezid, since it is so severe, even b'Shogeg and after Teshuvah, Kaparah is not complete without a Korban.