“And some men from Manasseh defected to David.” These are more men defecting from King Saul to David even before David was anointed king over Judah.
“David.” The Hebrew text says “him,” but the REV added “David” for clarity.
“master.” This is a grammatical plural; the Hebrew text reads “masters,” but it refers to King Saul.
“He will defect to his master Saul at the cost of our heads.” It seems that the Philistine leaders thought that David would buy back his favor with King Saul by turning on the Philistines and killing them. There is no “if” in the sentence although many versions put it in the text.