In The Adventures of Tom Sawyer, why did everyone think that muff potter killed the doctor?

In chapter 11, we are told that "a gory knife had been found close to the murdered man, and it had been recognized by somebody as belonging to Muff Potter." This then is the principal reason as to why everybody believes that Muff Potter is responsible for the murder.
We are also told that Muff Potter was seen washing himself in a small stream in the early hours of the morning, and that he was then seen sneaking away. The fact that he was seen washing himself is particularly suspicious given that Muff Potter was not in the habit of washing himself very often.
Muff Potter is also known around the town as a drunkard, and he also lives a life of crime in partnership with Injun Joe. These factors likely also play a part in the widespread belief that it was he who was responsible for murdering the doctor.
Muff Potter is also initially convinced that he, in a drunken stupor, really did kill the doctor. This is because Injun Joe put the knife in Muff Potter's hand while the latter was unconscious, and told him, when he awoke, that he was indeed responsible for the dead body in front of them.