In The Chrysalids by Parkes and Harris, what is the significance of the finding of Aunt Harriet's body?

Aunt Harriet's suicide demonstrates the effects of the harsh doctrine on the people of Waknuk. 

Aunt Harriet, David’s mother’s sister, had a baby around the same time Petra was born.  It was not her first baby, but she was never allowed to keep any of her children.  We are not told why.  All we know is that they do not conform to the True Image in some small way.  A baby has to get a Normalcy Certificate, and Harriet’s babies would not be able to. 

Since David’s mother has Petra, Harriet comes to her in desperation.  She wants to borrow the baby for a few days so she can get a certificate for her baby, because she wants to finally be able to keep one.  David’s father is horrified that she would want to keep the baby, which he calls a “defilement.” 

A baby which, if you were to have your way, would grow up to breed, and, breeding, spread pollution until all around us there would be mutants and abominations. That is what has happened in places where the will and faith were weak: here it shall never happen.  (Ch. 8) 

For whatever reason, Waknuk is convinced that if they do not maintain genetic perfection they will be doomed.  We know there was some kind of catastrophic event in the past that they are trying to prevent recurring.  David’s father tells Aunt Harriett that by trying to keep her baby she would betray them all. 

David notes that while his mother wasn’t prepared to break the “Purity Laws,” she did cry when her sister was sent away.  It must have been hard to turn away her own sister when she was so desperate, especially when whatever was wrong with the baby was apparently very minor.  David wonders “how many mothers there might be who were turning a blind eye towards matters that did not actually infringe the Definition of the True Image.”  He doesn’t understand his father’s insistence on maintaining perfection. 

Aunt Harriet leaves, telling them she would pray for them and a world that has no sympathy for the weak. 

When they broke the news to me next day that my Aunt Harriet's body had been found in the river, no one mentioned a baby.... (Ch. 8)

It is a sad story all around.  People who do not conform to the True Image are not allowed to continue to live in Waknuk.  This means that they are expelled to the Fringes, where they live a cursed life.  As Aunt Harriet's death shows us, those who are left behind suffer too.