This book made me so upset that I could not sleep last night. I guess it is to be expected given my backround of working with several dog rescue groups for the past 15 years. Why would anyone create such an evil scenario? A dog is truly mans/womens best friend. A dog will give total love and affection and ask very little in return. Maybe I have not read enough of Mark Twain but if the next one is like this, I will not be reading the rest of Mister Twain's works.
As I understand it, the whole purpose of the story was to make people feel that cruelty to animals is wrong. It's sort of like Uncle Tom's Cabin where horrible events are described but the descriptions are meant to compel you to sympathize with the person (or animal) being abused, not with the abuser.
So I don't think you should be upset about this. Twain was creating this horrific situation to make people sympathize with the dogs and push for humane treatment for animals.