Matthew Henry Commentary 2 Kings 9:11-15

"Then Jehu came forth to the servants of his lord: and one said unto him, Is all well? wherefore came this mad fellow to thee? And he said unto them, Ye know the man and what his talk was. And they said, It is false; tell us now. And he said, Thus and thus spake he to me, saying, Thus saith Jehovah, I have anointed thee king over Israel. Then they hasted, and took every man his garment, and put it under him on the top of the stairs, and blew the trumpet, saying, Jehu is king. So Jehu the son of Jehoshaphat the son of Nimshi conspired against Joram. (Now Joram was keeping Ramoth-gilead, he and all Israel, because of Hazael king of Syria; but king Joram was returned to be healed in Jezreel of the wounds which the Syrians had given him, when he fought with Hazael king of Syria.) And Jehu said, If this be your mind, then let none escape and go forth out of the city, to go to tell it in Jezreel." — 2 Kings 9:11-15 (ASV)

Those who faithfully deliver the Lord's message to sinners have in all ages been treated as madmen. Their judgment, speech, and conduct are contrary to those of others; they endure much in pursuit of goals, and are influenced by motives that others cannot comprehend. But above all, this charge is brought by all sorts of worldly and ungodly people, who are, in fact, mad; while the principles and practices of God's devoted servants prove to be wise and reasonable.

Some faith in the word of God seems to have animated Jehu to this undertaking.