Question of belief (Cornerstone)

As Venice experiences a debilitating heatwave, Commissario Brunetti escapes the city to spend time with his family. For Ispettore Vianello, however, the weather is the last thing on his mind. It appears his aunt has become obsessed with horoscopes and has been withdrawing large amounts of money from the family business. Not knowing what to do, he consults Brunetti and asks permission to trail her. Meanwhile, Brunetti receives a visit from a friend who works at the Commune. It seems that discrepancies have been occurring at the Courthouse involving a judge and an usher with a flawless track record. Intrigued, Brunetti asks Signorina Elettra to find out what she can while he's away. When news reaches Brunetti that the usher from the Courthouse has been viciously murdered, he returns to investigate. But why would someone want a good man dead, and what might his death have to do with the Courthouse discrepancies?