How were we all like Tamei?
Rashi: It is because Tzadikim were removed from us.
Radak (4): We are all like Tamei (without merit) - do for Your sake!
Malbim: (a) Now that You do not descend to do miracles for us, we are like a Tamei who is distanced from Kodesh. (b) A Tamei person is Metamei his colleague. Since we are all Tamei, there is no one to sprinkle Mei Chatas on us to be Metaher on us (make us repent)!
What is "uch'Beged Idim [Kol Tzidkoseinu]"?
Rashi citing Targum Yonasan: V'Chilvush Merachek - like a despised garment, that all say 'remove it.' Idim is the Targum of removal.
Radak #1: The Targum of "Sapachas" (Vayikra 13:2, a kind of Tzara'as) is Adya.
Radak #2: The Targum of "Harah" (Bereishis 16:11, pregnant) is Me'adya. When she gives birth, her clothes are dirty with blood.
Radak #3: It is a tattered, worn out garment. Chazal discussed 'one who uses Edim' (Eduyos 1:1 - a rag to check for blood). According to all the Perushim, a dirty garment is removed from people. We remained in Galus like a Tamei matter, or like a despised garment. So all our Tzedakos, even the Tzedakos that we do, are not accepted, rather, despised, for we do them only to boast and aggrandize ourselves. All is due to envy of others.
Malbim: Also the Mitzvos that we do are like a garment that us only for Adi (an ornament). We do them only for glory, and not to make a strong Kinyan in our Nefashos.
What is the meaning of "va'Navel ke'Aleh"?
Rashi: We will wither like a leaf.
Radak: Just like a leaf withers and falls, so we all fall. And just like wind carries a fallen leaf, so our sins carry us to strew us in every corner. Some say that va'Navel is Nif'al conjugation. It should say v'Ninavel; the Nun of the root is omitted. Some say that va'Navel is Hif'il. There should be a Dagesh in the Veis; there is Nach in place of the Dagesh, like in "va'Yazed Yakov" (Bereishis 25:29).
Malbim: The tree and the fruits do not wither, only the leaf, which is secondary and weak. So we do not have fruits of Tzedakah and good deeds.
Why does it say "va'Avoneinu ka'Ru'ach Yisa'unu"?
Rashi citing Targum Yonasan: In our guilt, we are carried about like a wind.
Malbim: Our sins carry us, like wind carries a fallen leaf. One who has Torah and good deeds is like a tree with many roots. All winds do not budge him from his place. If he is strong in his Emunah, the Ru'ach of desire does not lead him to sin, He is strong against the Ru'ach that puts evil pictures on the heart. Intellect is like a strong wall; this Ru'ach cannot pass it. However, since we are like a fallen leaf, we have no internal Emunah and Mitzvos, so our sins carry us via the Ru'ach that blows vehemently in the cracks of the Nefesh, and carries dust and earth to cover its sun.
