Sforno on Numbers 25:11/ספורנו על במדבר כ״ה:י״א

1בקנאו את קנאתו בתוכם, for having taken G’d’s vengeance in the presence of all the people, so that by their watching what he did and not protesting his deed they would qualify for atonement of their sin for not having protested the sinners at the time when they were about to commit their evil deeds. [compare author on verse 4 in this chapter where he made a similar point explaining the unusual command by G’d to Moses. Ed.]