Who thought that there is no Elokim?
Rashi: Titus. He stabbed the Paroches, and blood came out. He thought that he killed Him! (Gitin 56b)
Radak (14:1): The king of the nations that Yisrael are in their hands. Since he harms Yisrael and presses them, he thinks that there is no Elokim, no Judge and law in the world to pay a man according to his deeds.
Malbim: Sancheriv sent Sheluchim to blaspheme the living G-d.
Above, it says "Naval". Why does it say "Hishchisu" (plural)?
Radak (14:1): He and his nation [were corrupt].
What is the meaning of "v'His'ivu Avel"?
Radak: They made their way abominable via the evil that they did. It should say b'Avel; the prefix Beis is omitted, like "ha'Nimtza Beis Hashem" (Melachim II, 12:11).
Why does it say "Ein Oseh Tov"?
Radak (14:1): All corrupted their deeds. All their deeds are for evil, for they think that Hashem does not see.
Malbim: Sancheriv and his camp were all evil. They did not even follow etiquette!