I have a different answer. A the beginning of the first chapter in volume 2, Victor is located at his family home of Belrive. This is coincidentally the same place in which he witnessed the first effect of lightning during a thunderstorm. The family had retired to that home for a while in order to go through the grief and loss process after the deaths of Justine and William.
It was a bittersweet thing for Victor. He felt that he was finally free for once from the shadow of the monster, but that was not so. In fact, every time he realized what he had done he would feel even worse, especially now that he was isolated with his family. He himself admits to gnashing his teeth and wanting to commit suicide while in there. The guilt could not leave his mind, and he was deeply anxious throughout the whole time there.
At the end of Chapter Six, Volume One, of Mary Shelley's Frankenstein, Victor has finally returned to the man he was before he began his studies at the university in Ingolstadt—before he created the creature in his laboratory. He is filled with laughter and joy.
However, in the first chapter of Volume Two, Victor receives a letter from his father in Geneva. It informs Victor of the murder of his youngest brother, William, and Elizabeth's desolation at this loss. Victor is filled with despair over this loss of his beautiful and loving sibling. In his agony, he wants nothing more than to return home to be with his family.
However, before Victor reaches Geneva, waylaid by the closing of the town's gates at sundown, he comes upon a vision that makes all things clear to him regarding William's death. In the midst of a raging thunderstorm, between flashes of lightning, Victor sees the image of the monster he created walking on the landscape before him. For only a moment he wonders at the creature's presence. In the next instant, Victor is certain that it is the monster that has killed his brother. The anguish of this knowledge is compounded only by the realization that Victor is responsible for unleashing this villain upon the world, who has now taken his brother's young life.
Alas! I had turned loose into the world a depraved wretch, whose delight was in carnage and misery; had he not murdered by brother?...I considered the being whom I had cast among mankind, and endowed with the will and power to effect purposes of horror, such as the deed which he had now done, nearly in the light of my own vampire, my own spirit let loose from the grave, and forced to destroy all that was dear to me.