Final answer:
Victor feels immense guilt and is overwhelmed by the weight of his own conscience when Justine expresses pity for the person responsible for William's murder.
Step-by-step explanation:
When Justine talks about feeling great pity for the man who must carry the guilt for William's murder on his conscience, Victor feels immense guilt and is overwhelmed by the weight of his own conscience. This is because Victor is the one who created the creature that ultimately caused William's death, and he is tormented by the realization that he is responsible for the tragedy. Victor's guilt becomes even more pronounced when Justine is wrongly accused of the murder and is executed for a crime she did not commit, further intensifying his feelings of guilt and remorse.