The answer to this question can be found in Chapter Seven of this great novel, when Victor receives a letter from his father informing him of some terrible news. This of course comes as an even greater shock than before, because Victor is still reeling from the creation of his creature and how this has impacted him. Consider what terrible news this letter contains:
William is dead!--that sweet child, whose smiles delighted and warmed my heart, who was so gentle, yet so gay! Victor, he is murdered!
Victor thus learns that his younger brother, William, has been cruelly murdered under suspicious circumstances. As he returns home, he begins to think about the crime and links it in his mind to his own "crime" of the creation of his monster:
Two years had now nearly elapsed since the night on which eh first received life; and was this his first crime? Alas! I had turned loose into the world a depraved wretch, whose delight was in carnage and misery; had he not murdered my brother?
Victor then comes to suspect that it was his "monster" that he had made who was responsible for this monstrous act, and that the ultimate responsibility for this act therefore lies at his door, as it was he who made the creature in the first place.