אילמלי אתה שאול והוא דוד

משיח בן דוד and משיח בן יוסף have different roots. Yosef was the one who resisted temptation where as Yehuda was the one who admitted his wrong doing. Dovid too sinned to teach תשובה as Rashi points out.

This is perhaps what the Gemara means, besides for the judgment on the actual sin there is a much greater judgment as to whether a person is doing his תפקיד in life. This concept is learned from a medrash that when a person goes to the heavenly court 3 מלאכים greed him 1 to judge his positive actions 1 to judge his negative ones and the 3rd to see "where he is holding". this 3rd מלאך is checking to see if he did his job (see גר"א משלי ו' ל"ג). Shaul's essence was not to sin - at all! so even few sins caused his demise, Dovid on the other hand is suppose to reveal T'shuva and as such "get's a brake

this has a little to do with the last discussion, for there is a story of a Yeshiva boy who was stomped with not knowing a simple Tosfos over lunch. He got so upset about it and left lunch with out bentching and went straight to the bais medrash and went on to become a great masmid and a gadol. The question is will he get punished for not bentching or do we say that since he fealt this surge of energy will get him going to complete his תפקיד he is a sort of an אונס and may bend the rules. i heard 2nd hand that one of the famous American poskim in erets yisroel justified this boy's actions. to me that is strange.