Why did Injun Joe want to take revenge on young Dr. Robinson? Narrate how he settled the score and put the blame on Muff Potter?

When Muff Potter demands more money from the doctor, who refuses to pay, Injun Joe intervenes, threatening the doctor with his fists. "You done more than [pay us]," says Joe, recalling how five years ago Dr. Robinson had turned the Injun away from his door when he was asking for food. With revenge on his mind, Injun Joe shouts: "And now I've got you, and you got to settle, you know!"

Dr. Robinson is quick to strike Injun Joe to the ground, after which Muff Potter tackles the doctor to the ground. The doctor flings himself free and strikes Potter unconscious with heavy headboard of the grave. Seeing his chance, Injun Joe grabs the knife Potter had dropped during struggle and stabs the doctor in the chest. As the fatally wounded doctor falls over Potter, Huck and Tom run away in fright.

"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.