Yorkies, like all Terriers, were bred to hunt and kill vermin all on their own. As in, go down the hole where the rat (or whatever) lived, and fight and kill it. Without necessarily getting any help from a human. When a dog breed has been bred for that, then being stubborn, hard to convince, and downright mean is part of the job description!
(Dachshunds were bred to go down badger holes and kill badgers. Explains alot about Dachsies
) They tend to be diggers too. Especially when they're bored or otherwise not supervised.
They're supposed to be "scrappy". Some people apparently like that in a dog
They're "yappy" so that if a human is helping the human can find them underground, dig them up, and help.
That being said, if this was a dog forum and someone came on here saying "my family died and my brother wants me to get rid of the dog" how do you suppose people would react? For that matter...if she was the chicken person and you had the dog...would you be telling her to get rid of the chickens?