What happens at Justine's trial? How does Victor respond?

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.