Atticus says that Mr. Ewell was a coward and didn't have the guts to go after Atticus himself so he went after his children who are an easier target.
Atticus feels that Bob Ewell has desired revenge against him, but he cannot believe that Ewell would harm his children; on the other hand, Sheriff Tate sums up Ewell as a "low-down skunk with enough liquor in him to make him brave enough to kill children."
Heck Tate understands the truth of the situation, which is that Boo Radley killed Bob Ewell.