The slaying of idolaters appertains not to every particular man." {49c} This means that every Protestant king should massacre all his inconvertible Catholic subjects! This was indeed a counsel of perfection; but it could never be executed, owing to the carnal policy of worldly men.
In writing about "the office of the civil magistrate," Knox, a Border Scot of the age of the blood feud, seems to have forgotten, first, that the Old Testament prophets of the period were not unanimous in their applause of Jehu's massacre of the royal family; next, that between the sixteenth century A.D.and Jehu, had intervened the Christian revelation.
Our Lord had given no word of warrant to murder or massacre! No persecuted apostle had dealt in appeals to the dagger.

As for Jehu, a prophet had condemned _his_ conduct.

Hosea writes that the Lord said unto him, "Yet a little while, and I will avenge the blood of Jezreel upon the house of Jehu," but doubtless Knox would have argued that Hosea was temporarily uninspired, as he argued about St.Paul and St.James later.
However this delicate point may be settled, the appeal for a Phinehas is certainly unchristian.

The idolaters, the unreformed, might rejoice, with the Nuncio of 1583, that the Duc de Guise had a plan for murdering Elizabeth, though it was not to be communicated to the Vicar of God, who should have no such dealings against "that wicked woman." To some Catholics, Elizabeth: to Knox, Mary was as Jezebel, and might laudably be assassinated.

In idolaters nothing can surprise us; when persecuted they, in their unchristian fashion, may retort with the dagger or the bowl.