I guess kindness and mercy are not for the *untrainable* in South Dakota.
I believe she told this story in her book to show people that she had the grit to make tough decisions and to do the unpleasant things in life.
She writes that she hated the dog for misbehaving and spoiling her hunting trip but why kill it rather than find it a good home with a family somewhere?
Nothing in the situation needed to be tough or unpleasant.