Who Is heck tate actually protecting by saying the Ewell fell on his own knife?

By saying that Bob Ewell fell on his own knife, Heck Tate is protecting Arthur (Boo) Radley from the unwanted attention of the town. He knows Boo only intended to protect Jem and Scout, and that he is a shy, kind man. Boo would be very uncomfortable if attention was drawn to his actions.

As Scout says, it would be like killing a mockingbird, for it would bring pain to a kind-hearted man who only wants to help others.