What is "Ed l'Umim"?
Rashi: He is an officer over them. He rebukes and testifies about their ways to them.
Radak: This refers to Mashi'ach. He warns and testifies about you. This is like "Ha'idosi Bachem ha'Yom" (Devarim 4:26), "Ha'ed He'id Banu ha'Ish" (Bereishis 43:3), "v'Hu'ad bi'V'alav" (Shemos 21:29). Even though these and similar verses are warning, they are also testimony, for one who warns, he warns in front of witnesses, lest [the transgressor] contradict the one who warned. It says "Ha'idosi Bachem ha'Yom Es ha'Shamayim v'Es ha'Aretz" - it is as if He said in front of [Yisrael], I warn you, that they will be witnesses between us.
Radak citing Targum Yonasan: [I made him] a master over the nations.
Malbim: This refers to Mashi'ach, from Zera David, with whom I made a Bris. He will warn nations to return to the true Emunah.
Why does it say "Nagid u'Metzaveh"?
Radak: He will be a governor over them, and command them like a master commands his slave.
Malbim: He will be a governor over them, and command them about matters between nations.
