In Chapter 7 of Frankenstein, after Victor has returned from a wonderful outing on verdant hills under what he calls "a serene sky," he receives a letter from his father, Alphonse Frankenstein.  In this letter, the father tells Victor that his sweet child who delighted him with his smiles and congenial ways has been murdered on the seventh of May. 

On that Thursday, Alphonse, his niece, and two sons went for a walk in Plainpalais on a lovely evening.  The weather was so pleasant that they continues their walk farther than usual.  When dusk came, William and Ernest were not to be found as they had gone on ahead; finally, Ernest arrived, but no William.  Ernest told the father that William had run and hid in their game, but he was unable to find his brother. They searched until dark; then, they returned with torches.  At around five in the morning, Alphonse discovered his son, stretched on the grass, white and motionless with an indentation made by the  the murderer's finger was on his neck.  After carrying him home, Elizabeth begged to see William; when she examined the neck of the victim, Elizabeth exclaimed, "O God! I have murdered my darling child," and she fainted.

Later that evening, Elizabeth informs Mr. Frankenstein that William had begged her to let him wear a valuable miniature of hers  that was made of his mother, and, now, this picture is gone, having probably been taken by the murderer.

In the letter from his father, Victor Frankenstein learns that his little brother William is dead.  Alphonse describes the event as William being strangled by an unknown stranger while taking a walk, found strangled with the killer's handmarks around his neck.  William’s locket with their mother’s picture was missing.  Victor panics and assumes that the monster killed him, because he sees the monster when he returns home.  This incident demonstrates the unraveling of Victor’s life, and his disgust for the monster.