I am going to assume you are referring to the murder of Victor's brother William and who Victor believes to have committed the crime instead of Justine. He believes that the creature has committed these crimes. He believes that the creature has murdered his dear brother William and then framed poor innocent Justine for the murder by planting the evidence on her. He sees the creature on his trip through the mountains and immediately wonders "could he be (I shuddered at the conception) the murderer of my brother? No sooner did that idea cross my imagination, that I became convinced of its truth". For this he does feel guilt in himself, but not enough to confess and risk looking like a "madman" before the law, figuring Justine would still be convicted with only his wild story to free her.