I think it's interesting that everything you read about him, he's referred to as "census worker Bill Sparkman", or "part-time census worker Bill Sparkman." They never refer to him as "substitute teacher Bill Sparkman", or "local Boy Scout leader Bill Sparkman".
If you put a bunch of things together, complete with knowledge of the area combined with utter speculation, you can come up with the most plausible thing that happened, several of them, albeit it a pure load of speculative crap.
In the early 1990's, a single father of a young son just entering elementary school, he started volunteering as a paraeducator to help his son and others in the class room with their work assignments. After a while he realized that he was doing the same things that teachers do, so he decided to go to school to get a teaching certificate, which he did in the fall of 2007 via online courses from Western Governors University in Salt Lake City, UT. But, he was suddenly battling non-Hodgkin's lymphoma from a cyst that appeared on the side of his neck. The school invited him to speak at his own Commencement, but he couldn't fly due to the lymphoma, so he drove from London to Salt Lake City for his graduation in 2008. While waiting for a math teaching job to open up in the London area, he left his job as a professional Scout and he took on the job of part-census taker.
It all sounds so far so good, but the other stuff is a little weird. No mention anywhere of the mother, but that's because he never married and his son was his adopted son, who is now in his 20's. When Sparkman was the equipment manager at Vanderbilt University he earned the nickname "Fe-Fe", ostensibly because of his poodle-like curls in his hair. You would think that would be "Fifi", but it was "FE-FE" that he had on his vanity license plate. Weird.
They found him hanged from a tree, but his feet were touching the ground. He was also bound and gagged and wearing only his socks. His hands were bound in front of him, not behind him, his wrists together and the knot securing it at his fingertips. There was no evidence of something that he may have been standing on and it got kicked away, as if he could have hanged himself merely by attempting to kneel, or NOT hanged himself merely by standing up. One local London officer, off the record, encouraged reporters to look closely into the death of David Carradine, but would not elaborate.
Gagged and bound and naked does not have anti-Feds written all over it, despite the fact that he had it written on him. Anti-federal sentiments run long and deep in those hills, going back to the late 1700's when the government really screwed the entire Appalachian area, to the widespread search and destroy of moonshine stills, to the more recent encroachments upon pot fields and meth labs. But gagged and bound and naked has sexual undertones, and overtones, written all over it. The cause of death is asphyxiation, but the police have not determined if it was murder, suicide or simply an accidental auto-erotic asphyxiation. Was it "Fed" that was scribbled on his chest, or was it the beginning of "Fe Fe" and the second "F" was scribbled to be mistaken for a "D"? Hhhhmmm...
He was hanged in a tree in a rural cemetery. If he was killed by locals, they'd have just dug a hole and buried him there. But they didn't. If it was an anti-Fed thing, and for some reason they wanted to make a public spectacle out of it, a way way off-the-beat and path cemetery is not the way to do it. Clay County is so rural that I'm not sure there really is anyway to make a public splash. The nearest "towns" are places like Botto, Goose Rock, Elk Gap, I'm talking way rural. (It was found in the Hoskins family cemetery on Hoskins Cemetery Road, off Arnetts Fork Road, just west of Marcum, about a mile south of the Peabody Ranger Station (a place I have been) about 12 or 13 miles east of Manchester, south down 66 out of Big Creek, for those who are familiar with the area.) Everything about this case is weird, and it gets weirder and weirder.
The State Police and the FBI have been nearly dead silent. They said the death was caused by asphyxiation, but refused to say whether it was caused by the rope around his neck. They did say, however, that it was not a "hanging death", which means he could have asphyxiated due to the gag places in his mouth and then duct taped shut. His body was hanging from a rope, but the rope appeared to be one that merely held him up (as if on display) as his feet were on the ground.
Still, after all the speculating that means nothing, I think it's just like a couple of locals and one of my relatives, said, "He knocked on the wrong door." Which means don't go to remote and rural homes in the Daniel Boone National Forest in the middle of the pot harvest and knock on a door and announce that you're from the Federal Government.