A good Kashya!

But why did you ask from the Gemara. you ought to have asked directly from the Pasuk, where Nasan ha'Navi compares David's act to a rich man who stole a poor man's only lamb and ate it? And what's more, when David commented on his Mashal that the rich man deserved to die, the Navi retorted 'You are the man!'

Clearly, there are different levels of sin. Briefly, David did not perform a sinful act. But in thought, he did. Notwithstanding the magnitude of the Yeitzer-ha'Ra (that he brought on himself), he was guilty of desiring another man's wife. Even if legally she was not his wife at the time, for all intents and purposes, at that moment, Uri'ah was destined to take her back after the war.

Perhaps for another man it would not have been considered a sin at all (certainly not one which, at a certain level, carries with it the death sentence). But David Hamelech was a Tzadik, and Hash-m takes Tzadikim to task for the slightest infraction ('like a hairsbreadth'). Above all, remember that, as the commentaries point out, Midos is the basis of the Mitzvos, and in the realm of Midos, it seems, David did sin, as we explained.

To add to your question, the Gemara in Yoma (22b) quotes Rav Yehudah in the name of Rav as saying that David ha'Melech was punished for the sin of Bas Sheva by becoming a Metzora for six months.

To answer your question, see the Tosfos to which Yehoash O. refers, above. As Tosfos points out, Rav (on 56a) refers only to major sins. The act with Bas Sheva was not a major sin, because she was not an Eshes Ish at the time. The Gemara here asks only that David ha'Melech has another major sin -- he accepted Lashon ha'Ra; as the Gemara in Erchin (16a) teaches, one who speaks or accepts Lashon ha'Ra is considered as though he has spilled blood. Therefore, it is on the same order of magnitude as killing Uriyah.

Even though David's act with Bas Sheva was not as severe a sin, he certainly needed some form of atonement, as evident from the Gemara that you cite, and from the verses that discuss the death of Bas Sheva's child. Similarly, although David ha'Melech's sin of counting the Jewish people was not such a severe sin, as Tosfos points out, the verse tells us that he suffered a terrible punishment because of it.

The Gemara here leaves the question about David ha'Melech's acceptance of Lashon ha'Ra unanswered ("Kashya"). The Rishonim (see Rashi in Sanhedrin 72a, and Rashbam in Bava Basra 52b and 127a) write that when the Gemara ends with "Kashya," it is not like a Teyuvta, and it is possible to find an answer. Tosfos Yeshanim in Yoma (22b, DH ul'Divrei) answers the Gemara's question from the sin of Lashon ha'Ra by saying that that sin was also not such a severe sin, since David ha'Melech accepted the Lashon ha'Ra only because there was circumstantial evidence to support it, and therefore he was considered a Shogeg. Again we see that David ha'Melech suffered a serious punishment because of this sin, as the Gemara here tells us (he lost half of the Malchus), even though the verse does not consider it to be a major sin of the magnitude of the killing of Uriyah.

(b) I posed your question to my Rebbi, Rav Moshe Shapiro shlit'a. He answered that the verse that limits David ha'Melech's sins to Uriyah ha'Chiti was written after David ha'Melech passed away. The verse means that this was the only sin that remained un-atoned for, because the sin of Bas Sheva was atoned for in his lifetime (as was made clear in the time of Shlomo ha'Melech, as the Gemara says on 30a).

(This explanation of the Gemara does not seem to be consistent with Rashi. Rashi (DH Rak) says that we see from this Gemara that David ha'Melech did not sin with Bas Sheva, and not that David ha'Melech was forgiven for the sin of Bas Sheva.)

I understand this answer as follows. The Gemara in Yoma (22a) says that David ha'Melech was punished during his lifetime for the sin of Bas Sheva, for speaking Lashon ha'Ra, and for counting the Jewish people. The only thing he was never punished for was the sin of Uriyah, about which Hashem promised him that he would not be punished, as the Gemara here (56a) says. Anytime David ha'Melech was punished, the punishment he received in this world atoned for the sin so that it would not remain in the World to Come. In addition, when he was punished, he took it to heart and repented fully. However, since he was not punished for the sin of Uriyah, he was not able to do a proper repentance.

This approach also answers the question of Tosfos as to why the Gemara here does not take into account that he sinned when he counted the Jewish people.

(The reason why the Gemara does ask from the fact that David ha'Melech accepted Lashon ha'Ra is because David ha'Melech was not punished for that during his lifetime. Rather, his descendants suffered. The answer to the Gemara's question is that even though he was not punished, there was a punishment for it. Therefore, David ha'Melech did a greater Teshuvah for that sin (because he heard from the Bas Kol that there would be a punishment for it) than for the sin of Uriyah, for which no punishment in this world was ever administered.)

We may ask, however, how does Rashi know to explain that when the Gemara (on 30a) says that Hashem gave a sign that He forgave David ha'Melech "for the sin," that it was a sign that David ha'Melech was forgiven for the sin of Bas Sheva? Perhaps it was a sign that Hashem forgave him for the sin of counting the people, or for the sin of accepting Lashon ha'Ra!

Rav Moshe Shapiro shlit'a explained that the sign that his Teshuvah was accepted came when the Aron entered the Kodesh ha'Kodashim in the merit of David ha'Melech. It is clear that the sin that was forgiven was the type of sin that would have prevented the Aron from entering the Kodesh ha'Kodashim had it not been forgiven. The only sin that would prevent the Aron from entering the Kodesh ha'Kodashim is a sin involving Giluy Arayos. The reason for this is because the Aron represents the presence of the Shechinah, and we find that the sin of Giluy Arayos causes the Shechinah to depart from the Jewish people (Shabbos 150a). Moreover, the two Keruvim atop the Aron represent the "marriage" because Hashem and Kneses Yisrael (see Bava Basra 99a). Therefore, it was obvious to Rashi that the Gemara refers to the atonement for the sin of Bas Sheva, which involved Giluy Arayos.

M. Kornfeld