Why were he tom and huckleberry so frightened?

Tom and Huck witness the murder of Robinson. "That score is settled," says Injun Joe as he robs the doctor's body and then places the bloodied knife in Potter's (who is still unconscious) open hand. When Potter comes to, Injun Joe acts as if it is Potter who has stabbed young Robinson to death. Convinced that he has murdered the doctor, Muff Potter begs Injun Joe not to disclose the events of the night, and the chapter closes with the empty graveyard.