Charles Ellicott Commentary 1 Samuel 25:21

"Now David had said, Surely in vain have I kept all that this fellow hath in the wilderness, so that nothing was missed of all that pertained unto him: and he hath returned me evil for good." — 1 Samuel 25:21 (ASV)

Now David had said. —This verse and the following (verse 22) must be understood as a kind of parenthesis in the narrative. They express what David felt and, as it were, his justification in his own mind for the violent and vengeful act he was about to carry out.

His argument was that Nabal had indeed returned evil for good. For a long time, David’s band had faithfully guarded his vast, scattered flocks and had preserved them safely. Now, when David asked for a small favor in return, the insolent man repaid him by throwing in his face the taunt that he was a rebel and a runaway slave.