Although Harper Lee never spells it out clearly, the reader must assume that Bob Ewell died at the hands of Boo Radley. Jem and Scout identified the sound of their stalker as someone who "shuffled and dragged his feet;" later, Scout smelled "stale whiskey" on the body laying on the ground. It must have been Bob, drunkenly staggering along after the children. Bob must have also owned a knife, since Scout heard "metal ripped on metal," and the sheriff later found that
... a shiny clean line stood out on the dull wire. "Bob Ewell meant business," Sheriff Tate muttered.
It was Bob who "slowly squeezed the breath out of me" before "he was jerked backward and flung on the ground." Although Scout originally thought that Jem had come to her rescue, it was actually the first appearance by Boo, and soon Scout heard coughing and "a sobbing, bone-shaking cough" followed by a groan. Although the sounds could have come from Boo (who was unused to physical exercise and sickly from never seeing the sun), it was probably Bob that she heard, dying from the knife wound to his ribs.
Although Atticus later believed, while in a state of shock, that Jem might have actually killed Bob, it was almost certainly Boo, although there still remains the possibility that Bob actually did fall on his knife during his struggle with Boo. There is also the issue of the two knives--the kitchen knife that was sticking in Bob's chest, and the switchblade that Sheriff Tate produced from his pocket. Although Tate claimed to have taken the knife off a "drunk man," it was probably either Boo's or Bob's. It could be that both Boo and Bob carried a knife that night, but it is uncertain which knife belonged to whom. In any case, it was Bob's body that rested under a tree in the schoolyard, and Sheriff Tate would officially call it an accidental, self-inflicted wound.