Charles Ellicott 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)

Slew them.—This ruthless slaughter of Baal’s prophets, as a judgment on their idolatry and perversion of the people, equally belongs to the fierce righteousness of the character of Elijah and to the spirit of the old Law. (See, for example, Deuteronomy 13:6–18; Deuteronomy 17:2–7.)

The law was adapted (as in the terrible, crucial example of the slaughter of the Canaanites) to the “hardness of men’s hearts.” In the imperfect moral and religious education of those times, it did not recognize the difference between moral and political offenses punishable by human law, and religious sin or apostasy, which we have been taught to leave to the judgment of God alone. Moreover, it enjoined an unrelenting severity in the execution of righteous vengeance—a severity that would be morally impossible for us, who have been taught to hate the sin and yet spare, as far as possible, the sinner.

The frequent quotation of such examples by Christians—of which Luke 9:54 is the first example—is a spiritual anachronism. In this particular case, however, it should also be remembered that those slain were undoubtedly implicated in the persecution headed by Jezebel, and that Baal-worship was a licentious and perhaps bloody system. Elijah, presiding over the slaughter that dyed the waters of the Kishon with blood, felt himself the avenger of the slaughtered prophets, as well as the instrument of God’s judgment.