I guess it depends on the reader. Yes, these people were backward and prejudiced, but they were not necessarily all bad. I was not happy they all got wiped out, even if I was happy that the Sealands rescued David and he was able to be himself.
I would actually disagree with your question. I think the ending of this great novel is not actually both happy and sad. There is no reference made to sadness as David, Petra and Rosalind are finally shown to have arrived in Sealand. Instead, all there is is overwhelming joy and relief to have finally reached a place where everybody is like them and able to communicate with thoughts. Finally they have reached a location where they do not need to hide and they are accepted for who they are. In particular, Rosalind is shown to have taken off her "armour" know that she does not have to defend herself and protect herself from the Waknukians. Note what we are told about her character:
She turned her head. The under-Rosalind was in her face, smiling, shiny-eyed. The armour was gone. She let me look beneath it. It was like a flower opening...
Thus it is that this is a happy ending not just because of their arrival in Sealand but also in terms of David's relationship with Rosalind. Yes, there may be elements of sadness in that they were forced to leave two of their original group behind them in Waknuk, but at the same time the overwhelming emotion is one of happiness at the end of this story.
There was a happy ending in which the telepaths finally made it to Sealand. It was their goal and hope to reach Sealand and they managed to achieve it, which means that the novel did indeed have a happy ending. However, I feel that the negative aspects outweigh the positive ones in The Chrysalids.
If we look at the ending alone, the telepaths had to leave behind Michael and Rachel, and we do not know whether they ever made it to Sealand. Furthermore, many people had died by the end of the novel, especially in the battle scene where the Sealand craft dropped the plastic-like threads all over them. These deaths are certainly saddening, especially the pitiful death of Sophie.
If we were to weigh the entire book, I would say that there were more sad events than happy (or hopeful): For example- Aunt Harriet's implied suicide -Anne's suicide -Sophie's capture and death etc
i disagree. i believe there is an element of sadness too, as the true human nature of cruelty is shown at the ending.
The waknuk people blinded by their beliefs could turn their backs on three children who had lived all their lives in the community, and try to hunt them down. the element of sadness is enhanced as we know that joseph strorm, david's father, is also part of the troop that is hunting down david and gang. this sort of reflects the intensity and power of religion and beliefs, as it can make a father be willing to hunt down his own children just to adhere to his beliefs.
Also cruelty is yet again shown when the zealanders employ the tactic of killing the whole of waknuk society just to get to david, rosalind and petra. killing the waknuk society may be one thing, but the cruelty in doing so by using the thread-like things to let them struggle and kill themself, reflects the sad truth that human race can resort to extreme means to fight for their survival or just to reach their objectives.