Who killed Bob Ewell? Why is Heck Tate willing to cover this up?

Bob is patriarch of the notorious Ewell clan. Everyone in town hates the Ewells for being dirty, no-good "white trash." Bob's easily the worst of the bunch—a lazy, work-shy racist who regularly abuses his daughter Mayella. He puts Mayella up to making the false claim that she was raped and beaten by Tom Robinson. As Tom's a black man, virtually everyone in Maycomb assumes that he's automatically guilty, and so the court's formal verdict of guilty, when it comes, is depressingly predictable.
Bob Ewell sees the trial of Tom Robinson as an opportunity to be somebody—to pose as a fine, upstanding citizen who's only trying to protect his daughter's honor from a dangerous sex predator. However, Bob's attempts at making himself out to be some kind of hero fail miserably. Tom's defense lawyer, Atticus Finch, humiliates him on the witness stand, making him look like the ignorant, stupid liar he really is.
Bob vows revenge on Atticus and angrily confronts him in the street, spitting right into his face. Later on, as Scout and Jem are returning home from a Halloween party, Bob suddenly leaps out and attacks them. Being both lazy and a coward, Bob clearly has figured it would be easier to gain revenge on Atticus by getting to him through his kids. Fortunately, Boo Radley is on hand to save the day. Seemingly from out of nowhere, he stabs Bob to death with a knife and saves the Finch children from his evil clutches.
Atticus is convinced that Jem has killed Bob and starts preparing his defense. But Sheriff Tate knows what really happened. He knows that Boo killed Bob, but he decides to pretend that Bob's death was just an accident. In covering up the crime, the sheriff is trying to protect Boo. If the case went to court, then Boo would almost certainly be acquitted. As everyone in town hated Bob Ewell, his killer would be hailed as some kind of hero. (Ironically, this is the same status that Bob craved for himself.) Sheriff Tate knows that the gentle, quiet, and reclusive Boo simply wouldn't want the ensuing publicity, so he drops the matter. The official version of what happened that night will be that Bob Ewell accidentally stabbed himself when he fell over while carrying a knife.