Why did he say "Iti Tetzei"?
Malbim: You will not be among other soldiers, rather, with me. The king often chose foreigners to guard around him, lest they join with native soldiers who went to rebel 1 against him. He did not want David to go with the troops, lest David not want to fight against Yisrael 2 , or lest Pelishtim not trust him.
A foreigner is less likely to rebel. He has less connection with natives, and he has no hope to seize power over a different nation. (PF)
David said that he raids Yehudah and other places in Yisrael (27:10). Did Achish suspect that he lies? Perhaps he thought that David hates only certain places (e.g. those that informed on him), or he still loves certain places (due to friends or family there, even though his parents and brothers left their residence - refer to 22:3:2:1, 22:4:3:1). (PF)
