Zastrozzi is motivated by revenge. At age 15, Olivia, his mother, was impregnated by his father, Verezzi. Zastrozzi is his bastard son. Verezzi is probably mentally ill and definitely obsessed with God. He leaves Olivia in squalor to die. Before her death, Olivia begs her son to avenge her.
As an atheist, Zastrozzi is not motivated by any fear of retribution in the afterlife. With his partner-in-crime, Bernardo, and his on-and-off again lover, Matilda, Verezzi is tracked to a small town in Italy. The trio then plot to destroy him.
The questions that Zastrozzi deals with are those of revenge, justice morality, and how those things are defined outside of the code of religion and traditional law.