I Samuel 15:10-11 and 18-20

"Now the word of the Lord came to Samuel, saying."

"'I greatly regret that I have set up Saul as king, for he has turned back from following Me, and has not performed My commandments.' And it grieved Samuel, and he cried out to the Lord all night."

"'Now the Lord sent you on a mission, and said, "Go, and utterly destroy the sinners, the Amalekites, and fight against them until they are consumed".'"

"'Why then did you not obey the voice of the Lord? Why did you swoop down on the spoil, and do evil in the sight of the Lord?'"

"And Saul said to Samuel, 'But I have obeyed the voice of the Lord, and gone on the mission on which the Lord sent me, and brought back Agag king of Amalek; I have utterly destroyed the Amalekites.'"

(Saul shows here that some people genuinely think they're righteous or doing right in the Lord, but actually are failing, and need to be steered the right direction by other believers. Not everyone who claims to be a follower of Christ is as such.)