After observing that Justine's "pretended friends" are too cowardly to testify on her behalf and the public indignation is against her, Victor Frankenstein rushes out of court with an agonized conscience.  For, Victor has assumed that he need not confess because Justine, who is innocent, would not be found culpable of the crime.  However, Justine's innocence has nothing to do with the circumstantial evidence and the lack of support which does condemn her.

With "heart-sickening despair," Victor learns that Justine has been found guilty of William's murder.  If he were now to come forward, especially after Justin has confessed, Victor reasons that the judges will perceive him simply as mad:

...the whole world would believe me to be [mad], if I disclosed the object of my suspicions? 

Thus, Victor feels that it is hopeless for him to say anything.