Who does Victor think is guilty ?
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.
While forming his creature and even as the creature destroy's the Frankenstein family and seeks revenge upon Victor, Victor does not view himself as guilty. However, as Victor experiences the horrific consequences of his decisions and matures, he gradually takes responsibility for the creature and the damage it has done.
The following passage from chapter 4 exemplifies Victor's recognition of his guilt and the immoral nature of his actions.
"I then thought that my father would be unjust if he ascribed my neglect to vice or faultiness on my part, but I am now convinced that he was justified in conceiving that I should not be altogether free from blame. A human being in perfection ought always to preserve a calm and peaceful mind and never to allow passion or a transitory desire to disturb his tranquillity. I do not think that the pursuit of knowledge is an exception to this rule. If the study to which you apply yourself has a tendency to weaken your affections and to destroy your taste for those simple pleasures in which no alloy can possibly mix, then that study is certainly unlawful, that is to say, not befitting the human mind. If this rule were always observed; if no man allowed any pursuit whatsoever to interfere with the tranquillity of his domestic affections, Greece had not been enslaved, Caesar would have spared his country, America would have been discovered more gradually, and the empires of Mexico and Peru had not been destroyed. "
Victor notes that the recklessness and desire for power and glory that he embraced has been the cause of most tragedy in the course of human history.