What is Victor's theory about what happened to William?

Answer: Once Victor receives the letter from his father about his younger brother's death. He realizes that it was his creations doing. He then comes to realize that his creation found his family and came to find him.

Explanation: Victor tries his hardest to save Justine and recognize her innocence since he knows who the real killer is which he cannot reveal.

Justine ultimately decided to take the blame of the crime out of fear of going to hell and is falsely executed.