A theme in chapter 6 is the importance of tolerance of differences.
In chapter 6, David tries to come to terms with what has happened to Sophie. He feels guilty. He tries to explain to the other telepaths that a person with a deviation is not a monster. The inspector comes to visit him and questions him about Sophie. David says her deviation seemed minor, and she is not a monster. The inspector warns him that every part of the definition is important, and having five toes on each foot is part of the definition.
…[if] a child doesn't come within it, then it isn't human, and that means it doesn't have a soul. It is not in the image of God, it is an imitation, and in the imitations there is always some mistake. (ch 6)
He explains to David that he has “misplaced loyalty,” to Sophie, but also tells him that “Blasphemies are not treated the same way as Offences,” implying that Sophie may not have been killed. David is wracked with grief when he finds out that Sophie and her parents have been captured, and he tells Uncle Axel he wants to run away to the Fringes.
David is young, and trying to come to terms with his society's wrongs. As a child, he has his own moral code based on what he feels is right deep down. He knows that there is nothing wrong with Sophie. He regrets what he has done, not just because of any harm that came to her but because he knows that what happened to her is wrong. Difference is not blasphemy, and should be tolerated.