Samson’s vow basically amounts to, “This time I really have a reason to kill the Philistines!” But Samson already had good reason to destroy the Philistines: God had called him to do so! This underlines that Samson’s motivation for revenge here was that he did not get what his eyes wanted. So what happens is that once again, Samson now does the right thing (go after the Philistines) but for the wrong reason. Yet God will use his wrong motives to further wake Israel up from its comfortable relationship with the Philistines.
3 And Samson said to them, “This time I shall be innocent in regard to the Philistines, when I do them harm.”