The Amalekites hoped to sell the captured people as slaves.1 This would be more profitable than killing them. With David and his fighting men away to the north, it would have been easy to round up the women and children that were left behind. Most likely there were also a few men present among those captured.
2 and taken captive the women and all who were in it, both small and great. They killed no one, but carried them off and went their way.