Why does Sibyl resent Apollo in the novel, The Tyrant’s Tomb?

At the beginning of the novel, Sibyl's jar is a reminder of the curse Apollo foolishly made, leaving the Sibyl to exist for thousands of years with a body that decayed until she was nothing but a voice. She, along with Harpocrates, come to resent Apollo for everything he did to them, and that resentment builds up as long as the jar is closed. But once Harpocrates opens it; once both he and the Sibyl are free, they embrace peace in the afterlife by forgiving Apollo and letting go; something that men like Tarquin are never able to do.