Dovid mentioned openly that his complaint against Naval was his ingratitude, and his repaying evil to the person who did for him so many favors. We can suggest that not having Hakaras HaTov is the worst sin; as our entire Avodas Hashem is an expression of thankfulness to our Creator. (Chovos Halvavos). Especially Dovid, the sweet singer who wrote Tehilim (which is loaded with thankfulness to Hashem), i.e., he was the extreme opposite of Naval.
Avigayil mentions that: כ ש מוְֹּ כֶןְּ הוּאְְּּש"אְּ כהְּ כהְּ Naval’s very essence was ingratitude, and despite his great wealth, he was a terrible miser; the exact opposite of Dovid. Perhaps this is why she asked to be married to Dovid. The Gemara says that the Gan Eden which should have belonged to Reshaim (if they deserved it) goes to the Tzadikim.
Naval did not deserve Avigayil, and Dovid was worthy of her. According to Chazal, she was the greatest of Dovid’s wives. When the: נ ע ר lad reported to Avigayil that Dovid was ready to wipe out her house, he mentioned how good Dovid and his men had been to them. He includes that Dovid’s men had never embarrassed them.
ו ה אֲ נ ש יםְּטֹ ב יםְְּּל נוְּּמ אֹ דְּו ל ֹאְּה כ ל מ נוְְּּּש"אְּכהְּטו “They never disgraced us”. Notice how Dovid is so careful not to be M'vazeh disgrace people, just the opposite of Naval who was M'vazeh Dovid. And according to Mahar”i Kara, he said that Dovid is a slave who rebelled against Shaul, and he also comes from Rus HaMoavi'a.
