Belial, who was once loyal to Satan, has grown tired of the war between angels and its blurring of morality. Even Satan has taken more of shade of gray, and Belial wants to return to the black and white morality of earlier times. Satan opposes him because he wants to prolong the fighting and prevent a new Hell from emerging.
Belial's dissatisfaction stems from his former loyalty to Satan, who now seems to be embracing a more nuanced perspective. Belial longs for a return to the uncomplicated morality of earlier eras, whereas Satan is determined to maintain the status quo and prevent a new Hell from emerging.