Jamieson, Fausset and Brown on 1 Samuel 30:2

2. they slew not any, either great or small, but carried them away--Their conduct seems to stand in favorable contrast to that of David (1Sa 27:11). But their apparent clemency did not arise from humane considerations. It is traceable to the ancient war usages of the East, where the men of war, on the capture of a city, were unsparingly put to death, but there were no warriors in Ziklag at the time. The women and boys were reserved for slaves, and the old people were spared out of respect to age.