C. F. Keil and Franz Delitzsch on Numbers 25:8

Num 25:8

Through this judgment, which was executed by Phinehas with holy zeal upon the daring sinners, the plague was restrained, so that it came to an end. The example which Phinehas had made of these sinners was an act of intercession, by which the high priest appeased the wrath of God, and averted the judgment of destruction from the whole congregation (“he was zealous for his God,” ויכפּר, Num 25:13). The thought upon which this expression is founded is, that the punishment which was inflicted as a purifying chastisement served as a “covering” against the exterminating judgment (see Herzog's Cyclopaedia).Upon this act of Phinehas, and the similar examples of Samuel (1Sa 15:33) and Mattathias (1 Macc. 2:24), the later Jews erected the so-called “zealot right,” jus zelotarum, according to which any one, even though not qualified by his official position, possessed the right, in cases of any daring contempt of the theocratic institutions, or any daring violation of the honour of God, to proceed with vengeance against the criminals. (See Salden, otia theol. pp. 609ff., and Buddeus, de jure zelotarum apud Hebr. 1699, and in Oelrich’s collect. T. i. Diss. 5.) The stoning of Stephen furnishes an example of this.