Albert Barnes Commentary 2 Kings 10:18

"And Jehu gathered all the people together, and said unto them, Ahab served Baal a little; but Jehu will serve him much." — 2 Kings 10:18 (ASV)

Though we cannot ascribe a spirit of true piety to Jehu (see 2 Kings 10:29), we can understand well enough how the soldier, trained in the Syrian wars, revolted against the unmanly and voluptuous worship of the Syrian Goddess and wished to return to the simple, solemn service of Yahweh. It would have been dangerous to declare these views and feelings during Jezebel's lifetime. Even after her death, it was prudent to act cautiously, waiting until Ahab's party was crushed politically before broaching the religious question.

Having now slain all the descendants of Ahab in the kingdom of Israel and all the influential men of that party (2 Kings 10:7, 11, 17), Jehu felt that he could begin his reformation of religion. But even now he uses subtlety rather than open violence, declaring, Ahab served Baal a little; but Jehu shall serve him much.