Keep in mind Twain was also trying to portray an entire town, and
"Who" would of been too personal, especially when he was drawing the
focus onto a collection of people.  "That" is the best word for the
story told.  If the tale had been about the corruption of an
individual, then "Who" MIGHT of been better.

    Of course, it's possible that we're over analyzing this whole
thing, and Twain would be either amused and laughing at us to the
point of tears, or enraged to the point of using us as foolish
characters in his next story!