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.
In the book "Freak the Mighty" Killer Kane is Max's natural father. Max already feels like he odd because he is very big and struggles to learn. When he hears his grandfather refer to his father in that manner it also makes Max feel like he is a freak of nature since he is his father's offspring.
One of the things that bothers Max about his grandfather's statement is that he overhears his grandparents discussing how much Max looks like his father. His grandfather also talks about how Max is like his father. He even says that they had better watch out because they don't know what Max might do while they sleep.