Atticus asked Tate about the switchblade he was waving and Tate replied that Ewell used a kitchen knife. This seems to be the pivotal point that changed Atticus's mind regarding whether Jem killed Ewell.
Heck found the switchblade dropped by Ewell when he attacked Scout. He took the blade away. Boo came armed with a kitchen knife and killed Ewell. By taking the blade out of the equation Heck can claim Ewell was the one with the kitchen knife and, being the drunken sod that he was, fell on it killing himself.
The switchblade was significant because it was the tool Bob Ewell used to harm the Finch children. When Heck Tate took it from Bob to make it look as though he killed himself, the switchblade becomes a symbol of "letting the dead bury the dead".
To Kill A Mockingbird by Harper Lee
