Why does it say "Achen Chalyeinu Hu Nosei"?
Rashi: Achen always means 'however.' Now we see that it is not due to his lowliness. Rather, he is afflicted with afflictions.
Radak: Yechezkel wrote "Ben Lo Yisa ba'Avon ha'Av v'Av Lo Yisa ba'Avon ha'Ben" (18:10). All the more so, one will not bear another man's sin, and all the more so a nation will not bear another nation's sin. If so, why does it say "he trembles through our Pesha'im is crushed through our Avonos" (5)? The nations will say '[Yisrael] bore our sin.' It is not true, but they think so, when they see at the time of the salvation that Yisrael's Emunah is Emes, and the nations' belief was Sheker - "Sheker Nachalu Avoseinu Hevel
Radak: This is unlike "Avoseinu Chat'u v'Einam va'Anachnu Avonoseihem Savalnu" (Eichah 5:7). That is like "Poked Avon Avos Al Banim" (Shemos 20:5), when they continue in the fathers' deeds, like it says "l'Son'ai" (ibid). Eichah discusses people who lament; amidst their pain, their words are not precise.
Why does it say "va'Anachnu Chashavnuhu [Nagu'a Muke Elokim u'Me'uneh]"?
Rashi: We thought that Hashem hates him. It is not so. Rather, "he is profaned through our Pesha'im and crushed through our Avonos" (5).
Malbim: We thought that they deserve their illness and pain, from Elokim.
