Was Bob Ewell stabbed or did he fall on a knife?

Susan said: Boo Radley killed Bob Ewell with the knife that Ewell was going to use on Atticus did not want to live a lie protecting Jem from the killing, suggesting he truly If he fell on his knife, it's still possible to roll sideways, over, etc. There's always the possibility Jem killed him, or Bob got stabbed by his own knife.

Bob Ewell was killed by Boo Radley, who stuck a kitchen knife into his chest on the night of Halloween. Bob wanted to get revenge on Atticus for trying to kill the children who were walking back from a Halloween school party.