What prompted the sinner (Zimri) to do what he did?
Rashi (citing Sanhedrin, 82b): He did so when the members of his tribe approached him 1 and asked him why they were being judged with the death-sentence and he was doing nothing about it.
Since he was their Nasi. (Maharsha 82a
What does the implications of the phrase "Vayakrev el Echav es ha'Midyanis ... "?
Ramban (on Pasuk 5): Either to mutineer against Moshe and the judges and to sin publicly, or in response to the members of his tribe. 1
Rashbam: It implies that he took her to his brothers with the express intention of committing Z'nus with her. 2
Hadar Zekenim: The other sinners went to the women. He brought her to the Machaneh, and made his tent there.
Who was the "Midyanis"?
Rashi: The "Midyanis" was Kozbi bas Tzur. 1
As the Torah writes in Pasuk 15, and Tzur was one of the five kings of Midyan. See Rashi (on Pasuk 15.
Why does the Torah mention that Zimri did it "le'Einei Moshe"?
Rashi: He (grabbed the Midyanite woman her by the hair - Targum Yonasan, and) brought her his brothers in front of Moshe and asked him whether she was permitted (to marry). 1
Rashi: Because 'iF you (Moshe) say that she is forbidden, who permitted you to marry the daughter of Yisro?
By the Eigel, Moshe stood up to the entire nation. Why did he not kill the sinners here?
Targum Yonasan: Because he became angry and forgot the Halachah.
By the Eigel, Moshe stood up to the entire nation. Why was he so helpless here?
Rashi: Hashem caused him to forget the Halachah so that Pinchas should come and receive his just reward.