Saul’s oath was meant to convince Jonathan that Saul's plans to take David’s life had come to an end. This would be the fourth oath that Saul made (see 1 Samuel 14:1–52), a legitimate oath which he would break to incur God’s wrath (see Deuteronomy 23:21–23).
6 And Saul listened to the voice of Jonathan. Saul swore, “As the LORD lives, he shall not be put to death.”