Victor is profoundly guilty after the Monster that he himself created kills his brother William:
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.
Although it was the Monster who killed William, Victor knows that he is the one ultimately responsible for the death of his brother. The Monster isn't fully human; he lacks a moral compass and any kind of conscience. He therefore has an excuse for why he behaves the way he does.
The same cannot be said for Victor, however. He's most certainly a human, and a very intelligent one at that. He ought to have known that something like this was bound to happen sooner or later. But he was so obsessed with pushing back the frontiers of scientific knowledge and making a name for himself that he didn't stop to think through the...
(The entire section contains 3 answers and 948 words.)