1) The Mitzvas Aseh is not that of eating the Pesach. The Rambam (Hilchos Korban Pesach 1:1) writes that the Mitzvas Aseh is to slaughter the Pesach on 14 Nisan. In 1:2, he writes that if one deliberately let 14 Nisan go by without bringing the Korban Pesach (and was not Tamei or far away), he is liable for Kares.

2) Joseph, I think I know what might be bothering you here in the Sugya. This is what Rashi (7b, DH she'Yavo) writes. The Gemara (7b) states, "she'Yavo Aseh she'Yesh Bo Kares." This is referring to Korban Pesach. Rashi writes that if the Metzora will not be able to put his fingers into the courtyard, he will not become Tahor and will not be able to eat the Korban Pesach at night. The implication of Rashi is that the most important thing is that he should be able to eat the Korban Pesach, and since Rashi is explaining the Gemara that discusses Kares for not bringing the Korban Pesach, this would suggest that Kares is the punishment for not eating it. How does this reconcile with the verse in Bamidbar 9:13 which states, "And the man who was pure, and was not on a journey but still failed to perform the Pesach... will be punished with Kares... because he did not offer the Korban of Hash-m at the right time"? One learns from this verse that Kares is the punishment for not offering the Korban Pesach, and it does not depend on eating! Why, then, does Rashi on 7b, when explaining the Gemara which discusses Kares for Pesach, refer only to eating the Pesach and not to bringing the Korban?

3) I found, bs'd, on Otzar ha'Chochmah a Sefer on Yevamos called "Arim Nisi" by Rav Refael Mazuz zt'l, which helped me understand this Rashi. He points out that Rashi did not conclude that if he will not be Tahor at night to eat the Korban Pesach that he will be Chayav Kares. This is because he will not in fact be Chayav Kares at all. He is a Gavra d'Lo Chazi, a man who was unfit, at the time of bringing the Korban Pesach. Arim Nisi is referring to the Gemara in Pesachim 78b (first wide line) that says that according to Rebbi Nasan the verse (Shemos 12:4), "... according to every man's eating you shall make yor count for the lamb," teaches that for Korban Pesach we require a man who is capable of eating , a Gavra d'Chazi l'Achilah.

4) So it follows that if the Metzora, mentioned in Yevamos 7b, was not able to purify himself on Erev Pesach, he would not actually be liable for Kares because he is a Gavra d'Lo Chazi. He is not capable of bringing the Korban Pesach, and if he would try to do so the Korban would anyway be invalid. (In fact, the verse itself in Bamidbar 9:13, which I cited above, implies that if a person was unable to bring the Korban Pesach because he is Tamei, he is not Chayav Kares.) Arim Nisi concludes that what Rashi and the Gemara mean is that since the Mitzvas Aseh of Pesach is so serious that if a Tahor person did not offer the Korban Pesach he is liable for Kares, then we say that this very severe Aseh overrides an Aseh which does not carry Kares with it.

5) To summarize, a person is not liable for Kares for not eating the Korban Pesach, and a Tamei person is not liable Kares for not bringing the Korban. However, since the Mitzvah of Korban Pesach is so important, we try to do whatever we can in order that a person can become Tahor and bring the Korban and also eat it. Even though not eating the Korban Pesach is not Chayav Kares, it is still a big Mitzvah to eat the Korban Pesach.

B'Hatzlachah Rabah,

Dovid Bloom