Grande: The False Savior
As you can see in "Destruction Foreshadowed", Grande's leadership revolves around his promise to guide the warlocks to a new world away from the eventual destruction of the Magical Realm.
Yesterday, I realized that this was why he killed Sigma's father not when the latter called dark magic unethical but when the latter foretold that someone else than him would save the Magical Realm. This implies that Grande, like Kray Foresight from Promare, actually cares more about his desire to be seen as a savior than the lives of his own people.
The death of his father motivates Sigma to help Arusu, a little human girl and the true savior, in his own rebellion against the dictator whose true despicable nature only a few knows. As much as he is evil, I can't help but find Sigma's core motivation to be one of the purest in the series: a young child's love for his father.