Why does it say "v'Niglah v'Aznai"?
Radak: I know what they say - therefore, I swear
Why will this sin have no atonement?
Radak: It is worse than previous sins.
Sha'arei Teshuvah 1:47, 4:4: They did not heed the Nevi'im's words or fear them. They openly engaged in kinds of Simchah. This is Chilul Hashem, for which Teshuvah and afflictions suspend, and death cleanses.
What is "Ad Temusun"?
Rashi citing Targum Yonasan: This is a second death (Radak - of the Nefesh) in the world to come. Malbim - Ad Temusun can mean forever.
Radak: Galus will not atone for this sin. However, your death via the sword will atone for it. Chazal (Yuma 86a) learned from here that for Chilul Hashem, Teshuvah does not suspend [punishment], Yom Kipur does not atone, and afflictions do not cleanse. Rather, all [together] suspend, and death cleanses.
