John Gill Commentary 1 Samuel 14:44

"And Saul said, God do so and more also; for thou shalt surely die, Jonathan." — 1 Samuel 14:44 (ASV)

And Saul answered, God do so and more also A form of an oath imprecating evils upon him more and greater than he chose to mention, see the like form in (Ruth 1:17) , though Abarbinel thinks this is not the form of an oath, but an asseveration of a curse that would befall him; as that God would not answer him when he inquired of him, and that he would add to do so again and again, if he died not:

for thou shall surely die, Jonathan ; such words from a father must be very striking to a son, and argue a want of paternal affection in Saul, that could call his son by his name, and deliver such a speech unto him in so strong a manner.