"Alas! I had turned loose into the world a depraved wretch, whose delight was in carnage and misery." - Victor Frankenstein.
This line right here is where one can say that it's entirely Victor's fault, how can you know if the creature is a "depravad wretch" when you left him without doubt? I know that Victor's mental state was in shambles when he finally created the creature, but it was his responsability to be a parent to the form of life he created.
Victor can't simply accuse him of being neither a depraved evil nor a murderer because he doesn't know, and how could he know if Victor abandoned him at mere minutes of his birth.
Maybe the creature is who murdered William, maybe he didn't do anything at all and simply followed Victor. Who knows, but at the same time, Victor escaped from his responsability as a caretaker, and now those consequences will reach him.