The monster acts like an evil creature in the novel Frankenstein when he decides to avenge his creator, Victor, by killing all of his loved ones.
No matter how blindsided Victor had been, his wish to create a living being was a result of his hungry ambition, and not a pre-meditated nor evil plan to ruin someone's life. Victor had no idea what the consequences of his actions would be, and there is no way to judge him based on the gravity and the magnitude of what he did. In other words, Victor had no cannon to go by as to how to treat his creation.
The monster, however, was angry at Victor for having created him. He was also angry to be the way that he was, and to be alone in the world. Although his anger is justifiable, the reality is that his creation was a freak of nature, and so was he. Victor did not plan for that to happen. Yet, the monster bypassed this reality and chose a very cruel way to avenge Victor.
First, in what must have been a gruesome scene, he killed Victor's little brother, William. Killing a person is bad enough, but killing a child is unpardonable. It is attacking the truly innocent and that is indeed monstrous.
Second, with utter premeditation, he set up the child's caretaker, Justine, so that she would be blamed for the death of the child. She was found guilty, and eventually died in the guillotine.
Third, he killed Clerval who was Victor's best friend and childhood companion. Clerval was also an innocent person who had a lot to give to society. His death was indeed a huge blow to Victor.
Finally, when the monster killed Elizabeth on her wedding night, it displayed its biggest act of revenge and demonic anger. Why not just kill Victor and spare all the innocent people? What the monster wanted was to make Victor's life a nightmare, and he definitely achieved that. Yet, what this shows is that the monster had a bigger capacity for hatred than Victor. Victor might have made one huge mistake, but the monster caused the cruel and unnecessary deaths of multiple persons. That is enough to establish that the monster was indeed a true monster in the story.
In my mind, if this argument were going to be advanced, I think that one points to the taking of human life. The monster kills human beings. That is where the crux of the argument lies. Victor is pretty horrific, but he is not leaving a line of dead bodies and extinguished lives in his path. The monster loses a level of his humanity in taking the lives of innocent people. The fact that the monster seeks to frame Justine by placing William's locket in her hands while she sleeps is also quite "monster- like." His lack of humanity in seeking to put aside his desires for revenge and violence would make him akin to a monster, someone or something that threatens the fabric of a rational social order. I think that Victor has to bear some level of responsibility for what happens, but ultimately the carnage that is present is only there because of what Victor's creation does, which is where the argument for it being "the monster" might gain some level of traction.