Why does Heck Tate insist that Bob Ewell's death was self-inflicted? Chapter 30

In the book To Kill a Mockingbird Heck Tate has to be insistent because Atticus believes that his son, Jem had killed Bob Ewell.

Atticus finds that he can not live with himself or his son if he says one thing and does another so he states he wants his son to stand trial if he had killed Bob.  He is really fixated on the fact that his son had killed him.

Heck Tate knows that Boo had stabbed Bob, but to get Atticus to let go of the notion of Jem killing Bob he has to be adamant with Atticus.  In addition, he is aware that if Boo goes to trial he will not get a fair trial because people will not be any different towards Boo than they were towards Tom and another innocent man will be harmed.