1 Samuel 29:4
And the princes of the Philistines were wroth with him; and the princes of the Philistines said unto him, Make this fellow return, that he may go again to his place which thou hast appointed him, and let him not go down with us to battle, lest in the battle he be an adversary to us: for wherewith should he reconcile himself unto his master? should it not be with the heads of these men?
The Philistine princes demanded that David be sent back to the place appointed for him and not go down with them to the battle. Their demand was made in anger, from fear that in the battle he would turn against them and become their adversary. They reasoned that he could reconcile himself to his master Saul by killing Philistines, with the heads of their own men, even their leaders, to win back Saul's favor. They reasoned in this way while remembering his reputation for defeating the Philistines and fighting for his former master.
Their suspicion was that in turning against them he would be acting to protect Israel. Their protest kept David from going into battle against Israel, a course that would have made him a traitor to his people and to God.