Why is Charlie upset by the Colonels plans for Bertha and what does he do to thwart those plans?

The Colonel has decided to kill Bertha, who is Charlies favorite dog, because she is old and doesn't hunt well. Charlie begs the Colonel to spare her, but he tells Charlie that she is his dog and he'll do what he wants with her. Charlie steals Bertha and hides her in their father's old shed.