Why was Max worried about Grim calling Killer Kane "an accident of nature"?
Since Killer Kane was Maxwell Kane's father, Max was worried that he, too, may suffer from the same malady. Kane was a violent man who, we find late in the story, strangled Max's mother right before his eyes when he was a young child. Max's suffered from very low self esteem, in part from the knowledge of his father's past, and since he was told that he looked just like his father--as well as inheriting his huge size--Max was afraid that other aspects of his father would also be passed on. His grandfather, Grim, was also worried about these aspects, since Max showed signs of anti-social behavior before teaming up with Freak.
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