Victor Frankenstein creates his monster in his laboratory, but as soon as he does he realizes what horrific thing he has done.
Victor Frankenstein runs to his room but the monster gets into his room through his window!
The monster escapes into the woods, and he sees a human, that happens to William, Victor's brother.
So he saw this opportunity, and killed him.
Soon the news of William getting murdered reaches Victor through a letter from his family. His family is telling him to come back, so he must travel back.
On his way back, he sees the forest where his brother was murdered, and he happens to see the murderer, and Victor is more than 100% sure that his monster was the murderer.