Why are Joe and Tom troubled by their consciences?

In Chapter Eleven, word had spread through town about the murder of Doc Robinson. A bloody knife, identified as Muff Potter's, had been found close to the crime scene. It was said that one citizen had come across Muff Potter washing himself particularly suspicious because Muff was known for his lack of cleanliness ­ in the river, and it didn't help that he could not be found anywhere. Meanwhile, the entire town is gathered at the graveyard, including Tom and Huck, when Muff Potter unexpectedly returns to the crime scene. Confronted by the crowd as well as the bloody knife now in possession of the Sheriff, Muff breaks down and admits to the murder. Injun Joe, who is present, tells the listening citizens the tale of how Dr. Robinson was murdered, but lies and claims Muff committed the act of violence in a drunken rage. Both Huck and Tom are shocked when they realize that Injun Joe is lying, yet ignore their consciences and remain silent.

In the days following, Sidney begins to notice a change of behavior in his brother. Tom tosses in his sleep, keeping Sid awake with his nightmares. At school, Sid notices that Tom seemed to lose interest in all schoolyard activity, including the dissection of dead cats. Instead of playing with the other children, Tom would sneak away from the schoolyard to the jail cell where Muff Potter was held prisoner, smuggling small tokens and gifts through the barred cell window ­ an outlet to ease Tom's guilty conscience.