What misconception did Atticus have have a first about the was Bod Ewell was killed?

Atticus thinks that his son, Jem, killed Bob Ewell. The sheriff, Heck Tate, tells Atticus that Bob fell on the knife, therefore killing himself. However, Atticus refuses to accept what Heck says and believes he is only trying to "cover up" the killing. Later, when Heck explains that Bob was really killed by Boo (Arthur) Radley, Atticus accepts the sheriff's portrayal of Bob's death in order to protect Boo.
He thought Jem was responsible for Bob Ewell's death, and was afraid of the legal repercussions. The Sheriff Heck Tate eventually convinced him that Mr. Ewell killed himself by falling on his knife.