Why did Fern yell at her mother?

Fern yelled at her mother when she explained that her father was headed to the barn to kill the runt from the litter of pigs born the night before.

"Do away with it?" shrieked Fern. "You mean kill it? Just because it's smaller than the others?" "Do away with it?" shrieked Fern. "You mean kill it? Just because it's smaller than the others?"