1)
Why does the verse discuss their Simchah?
1.
Rashi: It is as if there is mourning in front of Hashem, and you eat and rejoice!
2.
Radak: They were not worried about Kel's words. They mocked and rejoiced!
2)
Why do they say "Ki Machar Namus"?
1.
Rashi: Nevi'im tell us, from Hashem, that we have no share in the world to come - let us enjoy our life here! Radak - this is Chilul Hashem.