What happens at Justine's trial? How does Victor respond?
Even though Justine is not guilty of the crime, she is convicted of William's murder. A piece of evidence was planted on her by the creature (he slipped it into her pocket). Victor knows that the creature committed the murder, but he allows Justine to be found guilty and executed anyway. Victor is afraid that if he tells the truth about the murder, he will draw the anger and disgust of his community and his family. He would have to admit to making the creature and then abandoning it. He is a coward and will not risk his own reputation. He does, however, feel very guilty about the fact that his creature killed his (Victor's) brother and that Justine takes the fall for the crime. Victor's guilt, though, makes him seem awfully self-absorbed; he claims that he suffers even more than Justine does. This episode is one of the prime examples of Mary Shelley's suggestion in the novel that men can be more "monstrous" than those we may consider monsters due to their horrid appearances.
At the trial, Justine is convicted of having killed William. She ends up being executed for the crime.
Justine is convicted, in part, because she confesses to the crime. She tells Victor, however, that she confessed only because a priest had been putting immense pressure on her to do so. The priest said she could be excommunicated if she did not confess and repent. So, afraid for her soul, she confessed.
Victor is devastated. He is not brave enough to come forward and tell what he has been doing (and admit the monster killed William). Quite rightly, he blames himself for the deaths of both William and Justine.