Figure Analysis
Jehu is a killing machine. Of course, he's God's killing machine—if that helps.
(His name means, roughly, "The Lord is He.") Elisha charges him with going on a rampage, killing all the remaining members of Ahab's house, including the current kings of Israel and Judah (Jehoram and Ahaziah) and Ahab's widow, Jezebel. Jehu fulfills this grisly mission with zeal, killing not only all of Ahab's family, but all the priests and worshippers of Baal. (He traps them at a giant ceremony where he pretends to be making offerings to Baal.)
However, by the time he's done killing everybody, he doesn't prove himself to be a model king. He, like everyone else in the northern kingdom, still has a soft-spot for the sins of Jeroboam. He keeps on worshipping the sacred golden calves Jeroboam had erected throughout the land. So, yeah—it's a little anti-climactic.