This depends to some extent on what you are talking about -- what crime. But I would say that both men are guilty of something. However, Injun Joe is guilty of much more than Muff Potter is.
Both men are clearly guilty of going out to rob the grave. That is why they are in the graveyard in the first place. But Muff Potter is not really guilty of much more than that. By contrast, Injun Joe is clearly guilty of killing the doctor. That is surely much more important than grave robbery.
Later on, we also find that Joe is guilty of theft and of at least planning to harm the widow. Muff is not guilty of these things either.
Injun Joe is guilty of murdering Dr.Robinson, not Muff Potter. Muff Potter was knocked out when the murder occured, and when he woke up, Injun Joe convinced him that he was the murderer. Muff Potter is guilty of nothing, he is the innocent.
injun joe is the one who is guilty of killing the young docter not muff potter