Why is Waknuk the most dangerous place in The Chrysalids by John Wyndham?

Waknuk is dangerous because it is a hotbed of religious intolerance.

The people of Waknuk believe that anyone or anything that does not conform to the True Image is an abomination.  The True Image is a narrow, dogmatic interpretation of religious mandates about what things should look like.  They have determined, for example, exactly how long a person’s arm should be.  Anyone who does not conform is expelled.  Animals and plants are destroyed.

It appears that David’s grandfather founded Waknuk as a place where he could enforce his particular brand of worship to God.

Elias Strorm came from the East, somewhere near the sea. … He himself maintained that it was the ungodly ways of the East which drove him to search for a less sophisticated, stauncher-minded region; though I have heard it suggested that there came a point when his native parts refused to tolerate him any longer. (Ch. 2) 

David’s father Joseph seems to have taken over the religious enforcement.  He is very strict and narrow-minded.  When David’s aunt had a baby that somehow did not conform to the True Image, he refused to acknowledge it despite the desperate mother’s protests.  His wife agreed. 

No one, indeed, would dream of mentioning the matter openly until the inspector should have called to issue his certificate that it was a human baby in the true image. Should it unhappily turn out to violate the image and thus be ineligible for a certificate, everyone would continue to be unaware of it, and the whole regrettable incident would be deemed not to have occurred. (Ch. 7) 

When a baby is born in Waknuk, it has to get a certificate called a Normalcy Certificate that declares it appropriately formed.  People without them are relegated to the Fringes, a dangerous savage area.  Part of the treatment is sterilization, so a person who does not conform to the True Image cannot breed. 

David befriends a girl with extra toes, Sophie.  Her parents have been hiding her.  However, eventually she is seen and sent to the Fringes.  David is unable to protect her.  His father beats the information out of him.

David and the other telepaths may look normal on the outside, and in fact have been certified as normal, but they are not.  They have the ability to read one another’s minds.  David’s Uncle Axel warns him to be careful, but eventually he and the others are found out and have to flee.