There are two offences in view. The first is the refusal from Nabal to let David share in the reward/profit from the work that he did. The second is the insult whereby Nabal labelled David as a runaway slave. Abigail, seeing as she is part of Nabal’s household, considered his offence to be her own and asked David to forgive them instead of taking his vengeance.
28 Please forgive the trespass of your servant. For the LORD will certainly make my lord a sure house, because my lord is fighting the battles of the LORD, and evil shall not be found in you so long as you live.