who really killed Bob Ewell and why?

Boo "Arthur" Radley killed Bob Ewell. It says that Bob Ewell fell on his 'knife'. Thus, Boo saved Scout and Jem.

You're close Davis. But not so close. Boo Radley didn't kill Bob. Bob killed himself with a kitchen knife. Thus, Boo saved Scout and Jem because of Bob's "not informal violence". Bob attacked Jem and broke his arm. Scout mentions in the beginning that,

"When Jem was thirteen, his arm was broken at the elbow." Thus, Davis's answer is partially correct but not incorrect.

Davis Aguilar-

You're correct!!!!!!