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.” – 1 Samuel 27:10
We read in 1 Samuel 27:10 that David deceived Achish concerning where he had attacked. The verse does not tell us whether this deception was for war or protecting the innocent from being killed.
Expository Notes of Dr. Thomas Constable
David used the opportunity that his location afforded to defeat and to annihilate the common enemies of Israel and the Philistines that lived to Israel’s southwest. David did not leave any survivors, as the Lord had commanded (Deuteronomy 3:18-20; Joshua 1:13). He was clearing the Promised Land of foreign foes so the Israelites could occupy it.
David walked a thin line of deception but was able to convince Achish that his victories were for the welfare of the Philistines. Really he was conquering Israel’s surrounding enemies, but he gave Achish the impression that his raids were against the southern portions in Judah.
David continued to subdue Israel’s enemy neighbors later when he became king (2 Samuel 8). Achish believed that David had alienated himself from the Israelites and would therefore be loyal to him from then on (1 Samuel 27:12; cf. 1 Samuel 17:9). 
John Dummelow’s Commentary on the Bible
10. Made a road] RV ‘made a raid.’ The south of the Jerahmeelites] Jerahmeel was one of the divisions of the tribe of Judah (1 Chronicles 2:9). The barren south was naturally named after the fertile lands on which it bordered: the’ south of Judah,’ ‘of Jerahmeel,’ and so on.
The deception was that Achish understood that David had smitten the Hebrew inhabitants of the lands bordering on the desert, whereas he had smitten the nomad tribes who dwelt in the actual desert. 
Ellicott’s Commentary for English Readers
(10) And David said, Against the south of Judah.—The answer of David to his sovereign lord, the King of Gath—for he was now, to all intents and purposes, a vassal prince of Achish—was simply a falsehood. He had been engaged in distant forays against the old Bedaween enemies of Israel, far away in the desert which stretched to the frontier of Egypt.
And from these nomads—rich in cattle and in other property, which they had obtained by years of successful plunder—he seems to have gained much booty, a share of which he brought to his “suzerain,” Achish.
But David represents that the cattle and apparel had been captured from his own countrymen, whose territory he was harrying. “The Jerahmeelites were descendants of Jerahmeel, the firstborn of Hezron (1 Chronicles 2:9; 1 Chronicles 2:25-26).
And therefore one of the three large families of Judah who sprang from Hezron.”—Keil, They dwelt, it is believed, on the southern frontier of the tribe of Judah. The Kenites were a race living in friendship with and under the protection of Judah.