John Gill Commentary 1 Samuel 27:12

"And Achish believed David, saying, He hath made his people Israel utterly to abhor him; therefore he shall be my servant for ever." — 1 Samuel 27:12 (ASV)

And Achish believed David
As he would have him understand his story; imposing upon him by ambiguous terms, insinuating he had been against the southern parts of Judah, when he had been against foreign nations that lay to the south of Judah:

saying, he has made his people Israel utterly to abhor him ;
they shall never forgive him the destruction of their cities, and the inhabitants of them, and the plunder of their goods and cattle; his name will be had in the utmost detestation and abhorrence, and he must never return thither any more:

therefore he shall be my servant for ever :
would be glad to continue with him as a servant, and be obliged to serve him faithfully and truly, since his own people, and even those of his own tribe, would never more receive him; it being, as he understood it, the south of Judah that he had been plundering.