What is a prophet? What is prophecy? It is a message from God, which may or may not be a prediction of future events. The prophet would be the one God gave the message to deliver. He is the delivery boy.
Saul at the very start did OK, being meek and shy when he was first chosen to be king. Not that far along he became more concerned with pleasing people than God. When caught in disobedience he tried to justify himself and/or to blame the transgression on someone else... such as the people.
"Now go and smite Amalek, and utterly destroy all that they have, and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass." I Sam 15:3
"But Saul and the people spared Agag, and the best of the sheep, and of the oxen, and of the fatlings, and the lambs, and all that was good, and would not utterly destroy them: but every thing that was vile and refuse, that they destroyed utterly.
Then came the word of the LORD unto Samuel, saying,
It repenteth me that I have set up Saul to be king: for he is turned back from following me, and hath not performed my commandments. And it grieved Samuel; and he cried unto the LORD all night.
And when Samuel rose early to meet Saul in the morning, it was told Samuel, saying, Saul came to Carmel, and, behold, he set him up a place, and is gone about, and passed on, and gone down to Gilgal.
And Samuel came to Saul: and Saul said unto him, Blessed be thou of the LORD: I have performed the commandment of the LORD.
And Samuel said, What meaneth then this bleating of the sheep in mine ears, and the lowing of the oxen which I hear?
And Saul said, They have brought them from the Amalekites: for the people spared the best of the sheep and of the oxen, to sacrifice unto the LORD thy God; and the rest we have utterly destroyed" I Sam 15:9-15
David was always a man of God even when he was a little boy. He made his mistakes too, perhaps on surface appearing as bad or worse than those of Saul. The difference was in his reaction when confronted with his error. He did not try to justify his error before God or God's prophet nor did he try to blame others for his mistakes.
"And David said unto Nathan, I have sinned against the LORD. And Nathan said unto David, The LORD also hath put away thy sin; thou shalt not die." II Sam 12:13
"And David spake unto the LORD when he saw the angel that smote the people, and said, Lo, I have sinned, and I have done wickedly: but these sheep, what have they done? let thine hand, I pray thee, be against me, and against my father's house." II Sam 24:17