why did heck tate want to let the dead bury the dead?

Atticus is a very rules oriented person. He will not accept a lie, even if it involves his kids. In the end Heck Tate is able to convince Atticus that a trial would do nobody any good. A trial would hurt Jem and certainly destroy Boo, the man who saved his kids. Atticus finally concedes that, in this case, the truth would do more harm than good. Atticus concedes that it is better to let people think Bob Ewell fell on his own knife and put this whole ordeal to rest.