What are the implications of the phrase "Hirgu Ish Anashav!"?
Rashi (citing Sanhedrin, 18a): It implies that each judge, of whom there were 78,600 1 , was ordered to kill two sinners.
Ramban #1, Moshav Zekenim #1: It implies that the Beis-Din (Sanhedrin, consisting of 23 judges) of each tribe was to kill the sinners of their tribe and the 1000 over which [each of them] was appointed. 2
Ramban #2 and Moshav Zekenim #2: Hashem in his mercy, told Moshe to instruct the judges to sentence the sinners to death, in order to stop the plague that was killing the innocent and the guilty alike. However when Pinchas sprang into action, 'prayed and stopped the plague,' 3 causing Hashem's anger to abate, the plague stopped and there was no longer any need for Beis-Din to carry out the death-sentence. 4
Ramban: They were not let off the hook however, as Hashem destroyed all those who sinned by Ba'al Pe'or before Yisrael crossed the Yarden, as the Pasuk specifically states in Devarim, 4:3, 4. (If there were witnesses and warning, Beis Din is commanded to execute! Refer to 25:4:4:2 and the first note there.
What is the difference between "ha'Nitzmadin l'Ba'al Pe'or" here and "v'Atem ha'Deveikim ba'Hashem Elokeichem" in Devarim 4:2, both of which have connotations of clinging?
Torah Temimah (citing Sanhedrin, 64a): 'You clung firmly to Ba'al Pe'or
Rashi writes that each of the 78,600 judges killed two sinners. If so, more than a quarter of Yisrael were executed, and we must say that the population had increased, for just after this, they counted 600,000 Yisre'elim!
Why is it difficult to say that the population had increased? They multiplied greatly in Egypt (a Midrash says, six children at a time), so there should have been a disproportionably high number of youths below 20 at the first census, all of whom were proper to be counted now! And according to Rashi (Shabbos 89b), that every wife become pregnant with a male after Matan Torah, even if no one else was born in the Midbar, the number of men should have been at least 900,000, even if there were only 300,000 youths at the beginning! (PF).