What does the incident with Angus Morton's horse tell you about people's general attitude towards deviation in The Chrysalids?
The people do not accept any deviation. Any unusual things are “Offences” and must be destroyed. However, they do make exceptions in some cases—such as Angus Morton’s horse.
There are “rumours of great-horses” that no one has seen. Angus tells everyone that they are certified.
My father was already uneasy in his mind at what he had heard of them, nor was the fact that Angus was the importer of them a commendation; consequently, it may have been with some prejudice that he went to inspect them. (ch 4)
When the inspector does come, there is some trouble with the idea of killing the horse because “the great-horses were costly creatures” (ch 4). It is interesting that the people are afraid to destroy an abomination that is valuable.
The impending crops were so orthodox that the inspector had posted only a single field, belonging to Angus Morton, for burning. (ch 5)
Since they won’t accept any differences, crops that are not just as expected are burned. The fact that perfectly good crops and animals are destroyed because they don’t fit someone’s idea of perfect is ridiculous.
Weeks of work gone up in smoke, pigs, sheep and cows gobbling up good food just to produce 'bominations. (ch 9)
Old Angus does get away something though.
Angus Morton's great-horses still around, an "officially approved" mockery of the Purity Laws… (ch 9)
Apparently Angus has gotten an exception, and he is allowed to keep his horse. This shows that there is some corruption in the purity standards, and they are at least somewhat aware that the restrictions are silly.