How can I relate this claim to the theme of Frankenstein: the creature is more human than Victor himself?
During the story, the creature has a realization about the nature of humanity, a realization that is certainly supported by his and others' actions in the text. Upon learning about the Native Americans who suffered a "hapless fate" at the hands of European settlers who came to the New World, he asks,
"Was man, indeed, at once so powerful, so virtuous, and magnificent, yet so vicious and base? He appeared at one time a mere scion of the evil principle, and at another as all that can be conceived of noble and godlike."
The creature's behavior, throughout the text, supports this notion of human nature. He is capable of both extreme good and kindness, such as when he saves the little girl from drowning, brings firewood to the DeLaceys to ease their burdens, and sympathizes with Safie and Felix and their sad story. However, he is also capable of malice and evil -- he kills William, he frames Justine for murder, and he murders Clerval and Elizabeth. In fact, one of the things that makes his later villainy so upsetting is that he truly did begin life as a benevolent and thoughtful being. In this way, however, he is truly more human than Victor because the creature embodies both of humankind's extremes.
Victor, on the other hand, really doesn't ever behave in a selfless or compassionate way. He thinks only of himself first; even when he destroys the mate he promised the creature and the creature vows to be with him on his wedding night, Victor neglects to consider the life of his bride and only thinks of his own potential death. He is incredibly self-centered. Victor wanted to create life for the glory of it, to be hailed as a creator of an entire new race. He doesn't ever do anything for someone else that wouldn't also benefit himself. In this sense, then, he is less human than his creature because he does not embody the two extremes that Shelley suggests are inherent to humankind.