Who does Atticus think caused Bob Ewell’s death? Why does Heck Tate insist that Bob Ewell’s death was self-inflicted? In what way is this...

1) Atticus believes that Jem is the one who killed Bob Ewell; this is why he is so adamant that the letter of the law be followed in documenting Ewell's death.  He doesn't want anyone to believe Jem was given special treatment because he is Atticus's son. 

Heck Tate understands the truth of the situation, which is that Boo Radley killed Bob Ewell.  Heck Tate also understands that to publicize this information would bring a lot of attention on Boo Radley, something Boo himself would never want.  To protect Boo's privacy, Heck Tate insists that Bob Ewell's death be reported as accidental; they will say that Ewell fell on his own knife.

It could be said that Bob Ewell's death was his own fault, since he died in an attempt to attack children.  Boo Radley was only defending Scout and Jem; had Bob Ewell left the Finch kids alone, he would not have died in this manner.