My gut instinct impression of this crime is that this was originally planned out as a robbery that went really, really bad. This would be far from the first crime where greedy criminals tried to extort money from an older wealthy couple.
1) a couple of thugs cruise in by boat late at night to avoid detection and security.
2) They demand money, and a whole lot of it from this wealthy retired couple.
3) The gentleman kindly tells these thugs that they can go F ** k themselves--they aren't going to get a damn thing.
4) In a rage, one of the perps pulls out a gun and shoots him in the head.
5)In a rare moment of clarity, one of the perps realize how stupid it is to leave a bullet in the skull that can be traced back to them. Out comes the machete. Of course this action could have also been done to terrorize the woman into cooperating with them.
6) The elderly woman is tied and gagged and hustled out to the boat that the men arrived in.
7) Still thinking they can extort money from this woman, they take her to god-knows-where and hold her in order to carry out the scheme. It may turn out that she actually went into a bank and withdrew thousands of dollars before anyone realized she was missing. It may even be plausible that they could try and demand a ransom from her family.............don't count it out yet. The cops are not revealing these details.
Many times cops will withhold important details of a crime, details which, if the public were made aware of them, might aid in apprehending the criminals. Why do the cops withhold important information like this ?? Nobody knows !! ......maybe they just like having little secrets amongst themselves. Maybe it has something to do with big egos.
The scenario shown above is just my theory.....we'll see how it plays out.