Joseph Strorm is obsessed with finding and routing out any genetic changes that are not considered normal within his society's population. The people that have genetic changes that are too drastic are called Deviants, and they are supposedly an offense against God himself and his creation.

So I learnt quite early to know what Offences were. They were things which did not look right—that is to say, did not look like their parents, or parent-plants. Usually there was only some small thing wrong, but however much or little was wrong it was an Offence, and if it happened among people it was a Blasphemy—at least, that was the technical term, though commonly both kinds were called Deviations.

As a leader of the community, Joseph Strorm especially feels it is his duty to succeed in his mission of keeping the community genetically pure. Strorm may have always grown up to be this kind of person; however, there is the possibility that he is so critical of possible changes and looks so closely at everything because of his experience with the spider-man. It turns out that the spider-man is Strorm's brother who was initially accepted as normal and later deemed a Deviant to be eliminated from the gene pool. The fact that his brother's Deviant genetics remained "hidden" could have been especially insulting to Strorm, and that is why he looks at everything so carefully.

It is only in Chapter 14 that we find out the true identity of the "Spider Man". It is when David, Rosalind and Petra are captured by the men of the Fringes that David comes face to face with the leader of the Fringes, who he has worked out is his uncle. Note the similarity that is drawn between David's father and his uncle:

A figure seated on a stool just inside the entrance looked up as we approached. The sight of his face jolted me with panic for a moment - it was so like my father's. Then I recognised him - the same "spider-man" I had seen as a captive at Waknuk, seven or eight years before.

During the dialogue that occurs between the spider-man and David, his nephew, the story comes out. David tells him:

"My father had an elder brother," I said. "He was thought to be normal until he was about three or four years old. Then his certificate was revoked, and he was sent away."

Looking back at David's father's actions, this explains a lot. Having had a "mutant" discovered in his own family as a brother, it is understandable why he is so intent on discovering and disposing of other mutants. The fact that his own very brother was a "mutant" who remained undiscovered for four years would have made him all the more desperate and urgent to root out any form of impurity everywhere - including in his own family.