My revised Theory –
Both Mr. and Mrs. D were originally going to be taken and held for ransom/extortion or as a warning to “keep quite”. (About what I have no idea) Mrs. or Mr. D resisted and the perp ended up hitting them over the head, killing them.
Now the ‘plan’ needed to change in mid stream. Mr. D was beheaded. The head was removed and disposed of somewhere else and the headless body was left behind to make it look like a demented serial killing and Mrs. D’s body was put in the lake to make it look like she had been abducted. All to totally confuse LE and to basically divide LE in their operational tasks and allow more time for the perp to get back to their other life.
The planning or active parts of the crime; finding and accessing the property, not being seen, being able to leave un-noticed, not leaving a huge messy crime scene with easy evidence, or stealing anything was very well thought out and organized, but the thought process ended up being flawed; pep did not anticipate that the D’s, because of them being older, would resist at all. This change in plans made the crime appear as if was for two different conflicting reasons.
A ransom/extortion plan could very well mean that someone has insider knowledge (or thought they did) of finances and was under the impression that it would be easy money and readily forthcoming. Also possibly for a re-payment for something that they felt they had been cheated out of or to force a decision about future finances.
A “keep quite” warning would still potentially be plausible. If that was what this was about from the start, then with S&R dead; problem solved, even if the actually killing part was non-intentional. If it was not the elder D’s themselves that the warning was originally for, then the warning is still viable and the perp can still achieve what he wants.
Who are the ones suppose to keep quiet, and about what, then becomes the next question.
The perp would not necessarily need to be anybody close to the elder D’s. Anyone who at all “appeared” somewhat familiar, at least by sight, would most likely not have aroused any suspension until it was too late.
It would also not need to be someone who lived in the community, just someone who could look and act like they knew the D’s. It would not take too much work to determine what the D’s routine was. Look at the information that is available right on the net about the area. The church has an extensive website with newsletters available to read through, and the problems within the HOA was viewable to anyone who looked it up. The layout of the house and the property lines are public information. Now add a Google Earth program in the mix and you can see the entire area up close. The knowledge of how to use a nautical chart of the lake and one trial run with some basic navigational skills would allow someone to use dead reckoning to maneuver on the lake at night without having to be familiar with the lake at all. The key would to be able to use the same boat or small differences in size, draft, prop dimension and calibrated speed tends to have an accumulated effect, and if not compensated for, mistakes can be made and you miss your target.
I still believe that this horrific crimes motivation was for financial reasons in some way; either to gain more or keep what they have. If it was not for money, then it was to insure that something remained unknown.