Albert Barnes Commentary 1 Kings 18:40

"and Elijah said unto them, Take the prophets of Baal; let not one of them escape. And they took them; and Elijah brought them down to the brook Kishon, and slew them there." — 1 Kings 18:40 (ASV)

Elijah required the people to show their conviction by acts—acts that might expose them to the anger of the king or queen, but which, once committed, would cause them to break with Baal and his worshipers forever.

Elijah is said to have slain the “prophets of Baal” because the people killed them at his command. It is difficult to explain why they were brought down to the torrent bed of the Kishon to be killed. Perhaps Elijah’s objective was to leave the bodies where they would not be found, since he knew the coming rain would send a flood down the Kishon ravine and carry the corpses out to sea. Elijah’s act is justified by the Law’s express command that idolatrous Israelites were to be put to death, and by the right of a prophet under the theocracy to step in and execute the Law when the king failed in his duty.