Murder doesn't come into play in Frankenstein until Victor learns from his father's letter that his brother William was murdered. In response to that murder, Justine Moritz is put to death - who was innocent of the crime. From this point on, murders continue to punctuate the horrors of Victor's creation as Clerval and Elizabeth are also murdered by the monster. After each murder, Victor considers his role in causing it, and as time progresses his argument for himself being the true murderer grows stronger.
Victor, upon learning that Justine Moritz is the accused of William's murder, proclaims her innocence, and says that he knows the true murderer. He places indirect blame on himself, but he doesn't tell anyone the circumstances that he knows led to William's death. He speaks to himself (and to Walton, years later) when he remarks, "I, not in deed, but in effect, was the true murderer."
Though the murders in Frankenstein seem numerous, there is actually quite a gap before the word comes up again when the Monster recounts his tale to Frankenstein. But as Frankenstein and Clerval journey to England, there has only been one murder by the monster, and one by the state (though Victor claims indirect responsibility for both).
Upon seeing Clerval's dead body, Victor exclaims (this time out loud), "Have my murderous machinations deprived you also, my dearest Henry, of life? Two I have already destroyed; other victims await their destiny..." In this passage we can see the progression of Victor's argument that he is to blame, directly for the murders. In the first question Victor asks above, he still places the monster in between himself and the crime. But in the next sentence Victor openly cops to murders: "Two I have already destroyed." Though it seems a direct confession, Victor still hasn’t told anyone the names of those he believes he is responsible for.
In the language of this passage we can also see what could be causing Frankenstein to place direct blame on himself. He mentions his "murderous machination." Shortly before this episode Victor had been working on a mate for his monster, in order to prevent murders like this from happening. But ultimately the thought of children coming from the monster and his new lady-creation worried Frankenstein too much for him to honor his agreement. By violating the agreement, Victor may have saved mankind from a proliferating race of monsters, but he also sealed the fate of those closest to him.
(Perhaps there is another reason that Victor’s near creation of another monster spurred on a change in his placement of blame: an interesting side-note, though a thorough discussion goes far beyond my capabilities…what is the role of God in the murders of his creation, in the Ruins of Empire? I’ve never read Paradise Lost, but I would be interested to know how God felt about the destructive wars fought by his creation.)
When Frankenstein's father comes to Ireland to retrieve him, Victor, for the first time, openly admits his guilt to another person. He admits to the murder of "Justine, poor unhappy Justine, was as innocent as I, and she suffered the same charge; she died for it; and I am the cause of this –I murdered her. William, Justine, and Henry –they all died by my hands." In this statement, Frankenstein becomes even more direct in his confession to the murders. Upon seeing Henry, Victor lamented "murderous machinations", but here he directly says (out loud) "I murdered her"…about as good a confession as any murder detective could ask for.
After the murder of Elizabeth, and Victor's father's death soon after, Victor goes to the magistrate to ask for his help in apprehending the monster. This is the first time (that I saw) that Victor tells his full story to another human. But an interesting shift occurs here. Victor, who has increasingly blamed himself for the murders, now reverts back to blaming his creation and not himself. He tells the magistrate "this is the being who I accuse" in reference to his monster and not himself. It's also interesting to me that the magistrate doesn't once think to place the blame on Victor (possibly due to the elevated social class to which they both belong). I think it is possible that Frankenstein lied in this moment, or at least wasn’t full with the truth. If he were to openly blame himself, the magistrate wouldn’t consider his call for help (which he does deny). And after this ruse fails, Victor commits himself to his end – either he or the monster will be dead, and that’s all that matters.
A trial by combat “settled accusation in the absence of witnesses or a confession in which two parties in dispute fought in single combat; the winner of the fight was proclaimed to be right.” William, Justine, Clerval, and Elizabeth. The only witnesses and confessors available for their cases are Victor and his monster. Victor desires this combat, but the monster alludes him to the end. We never see the outcome of such a trial, and therefore it will always be difficult to assign blame for the murders of the novel.