Jamieson, Fausset and Brown on 1 Samuel 30:11-15

11-15. they found an Egyptian in the field, and brought him to David--Old and homeborn slaves are usually treated with great kindness. But a purchased or captured slave must look to himself; for, if feeble or sick, his master will leave him to perish rather than encumber himself with any additional burden. This Egyptian seems to have recently fallen into the hands of an Amalekite, and his master having belonged to the marauding party that had made the attack on Ziklag, he could give useful information as to the course taken by them on their return.