The shredder empty on gas? Empty coffin?
SPOILERS*
They said the judge ruled the shredder death an accident, yet the police never mentioned in court that the shredder was switched off upon discovery, yet the mailman knew? That's suspect right there.. Oh well it adds to the plot I guess.
The coffin did contain Bubba's remains, but when first seeing that scene, he jumps out of fear as if it's empty "it's him!" like Bubba is the killer since he's not buried, but then the mail man says we will go to the cops, we have nothing to hide he's still buried..
So it was Bubba's ghost driving/operating machinery, and filling the suit up, as we see him walking into the shed before he kills the fat farmer in the silo. Great movie, story and acting, but IMO it's a who-dunnit, great to see once but wouldn't want to see again for a while, similar to The Machinest, as a who-dunnit.