2) Boo

3) He thinks Boo did it, but the Sheriff guy tells him he fell on his own knife.

4) Boo really kills him but the people were led to believe he falls on his own knife.

(Sorry for all the editing)

2) Boo Radley

3) Atticus did NOT think it was Boo; he thought it was Jem and that the sheriff was lying to cover up for Jem.

4) It was really Boo who killed Bob Ewell. The sheriff said that Bob Ewell fell on his own knife to cover up for Boo. Because Boo would not like to be dragged into the limelight. You can see that part in the last section of the book, although they did not specify it was Boo. =)