We read of a strange turn of events in this chapter. David knows he cannot trust Saul and that Saul will continue to search for him until he is killed or captured. So David turns towards his enemy, the Philistines, for help. Not just any Philistine, but the king of Gath, the land from where the giant Goliath had come from.
The Gershurites, Girzites, and Almalekites were enemies of both the Philistines and the Israelites who lived between modern day Israel and Egypt. They would often send up raiding parties against Israel and Philistia. David would attack them, kill every person, and return with the plunder. He would then lie to the king to make him think that he had attacked part of Israel.
The Lutheran Study Bible has an excellent note at the end of the chapter “The writer makes no indication that the Lord encouraged or approved of David’s behavior. Rather than living ambiguously, live boldly and clearly for your Lord, entrusting your future to His care rather than to your cleverness or the world’s favor. The Lord’s favor is for more precious.” I couldn’t have said it better!