1)
Why should He pay them Shiv'asayim?
1.
Orchos Tzadikim (Sha'ar ha'Anavah): If one speaks Leshon ha'Ra about another, he receives the other's Aveiros, and his own Mitzvos are given to the one about whom he spoke.
2.
Malbim: Considering the level of the insulter and the insulted party, it is not enough to pay according to their insult. They should be paid Shiv'asayim, i.e. many times without number.
2)
How did they insult Hashem?
1.
Radak: They said "Aye Elokeihem" (verse 10).
2.
Orchos Tzadikim (Sha'ar ha'Anavah): Insulting a Tzadik is like insulting Hashem; we find that Yisrael's enemies are called Oyvei Hashem.
