Albert Barnes Commentary 1 Samuel 20:2

"And he said unto him, Far from it; thou shalt not die: behold, my father doeth nothing either great or small, but that he discloseth it unto me; and why should my father hide this thing from me? it is not so." — 1 Samuel 20:2 (ASV)

It is not so - Jonathan’s unwillingness to believe evil of his father is one of the many admirable traits in his character.