When David raided the Geshurites, the Girzites and the Amalekites why did he kill everyone in the towns?

ANSWER

so no one could inform the Philistines about what he was doing

1st Samuel 27 :

8 Now David and his men went up and raided the Geshurites, the Girzites and the Amalekites. (From ancient times these peoples had lived in the land extending to Shur and Egypt.) 9 Whenever David attacked an area, he did not leave a man or woman alive, but took sheep and cattle, donkeys and camels, and clothes. Then he returned to Achish. 10 When Achish asked, “Where did you go raiding today?” David would say, “Against the Negev of Judah” or “Against the Negev of Jerahmeel” or “Against the Negev of the Kenites.” 11 He did not leave a man or woman alive to be brought to Gath, for he thought, “They might inform on us and say, ‘This is what David did.’” And such was his practice as long as he lived in Philistine territory.
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