Here's why:
* The Creature confesses: The Creature, who Victor created, confesses to killing William. However, Victor initially doesn't believe him and doesn't go to the authorities. He is driven by fear and a desire to protect his secret.
* No legal investigation: The novel focuses on the internal struggles of Victor and the Creature, not a formal investigation. There is no evidence other than the Creature's confession, which Victor discards.
* Justice is not the focus: The novel explores themes of creation, responsibility, and the consequences of playing God. The focus isn't on finding a criminal in a legal sense, but on the moral and psychological turmoil of both Victor and the Creature.
While Victor doesn't discover the murderer in the traditional sense, the truth is revealed to him. This revelation, however, doesn't bring him closure, but adds to his guilt and despair.