How does Victor's younger brother die?

William, Victor's youngest brother was murdered. While on an evening walk with the family, the boy disappeared; he was found dead the following morning. On the day of the murder, Elizabeth had allowed the boy to wear an antique locket bearing Caroline's picture. Upon examining the corpse, Elizabeth finds the locket gone; she swoons at the thought that William was murdered for the bauble. She comes to blame herself for his death.