Nabal acted as if he did not know who David was and acted disrespectfully within the parameters of that culture. Who didn't know who David was. Abigail risked her life by going secretly to David; more so returning back to Nabal. His heart turned like stone. There are plenty of men like him that are simply harsh and foolish to their wives even though their wives have saved them from tribulation and sword. Good deeds are rewarded with scorn and brutality for no other reason than that is just who they are; Son's of Belial, worthless/destruction.
I think that David knows even as the words fall from his lips that Saul will not be even minded for long. For Saul, every step he takes against David becomes more and more an embarrassment but, he is the last to admit that he himself is the cause. We see two men today very much the cause of their own problems. David in turn leaves while pondering the possibilities chooses to reserve retribution to God's hand in both cases.
2012 - pBiblx2 Field Wise Bible System Version 2.0.9d - GPL3