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.