I believe the person who did this crime is a serial killer with extremely unusual and violent methods. I also believe he did the Tara Baker crime in Athens and the Katie Janice crime in Atlanta. So this is what I think happened, based on how he would operate. The perp had already surveilled this couple before. Perhaps he saw them because he was frequenting the resort community there. For work or personal reasons. He had a thing for older women. So he chose them. So he had deactivated their security alarm and the cameras at the entrance to the complex the week before. Had already been inside the house. Possibly figured out the boat too. He was well prepared.
It was Friday, evening time, and the husband was already upstairs getting ready for bed. The wife was downstairs reading or doing the crossword puzzle.The perp parks a distance away near lake. He got in a boat that was from a random neighbor that he had already prepared. And made his way towards the house. He got into the house through lock picking. He surprised Shirley at the kitchen table or lv sofa and hit her on the back of the head. She screamed. The husband ran down to try to defend his wife. But he saw he was losing and so the husband ran out of the house. The perp followed him and shot him in the back of the head to prevent him from getting away. He had not planned on this happening. The perp drug the body of the man back into the garage. Putting towels under his head. He then went back to the kitchen and sexually assaulted the half stunned wife. He put her clothes back on after to make it look like he didn't sexually assault her. I think maybe he tied her up. Then he went out to the garage and cut the husband's head off. To hide the bullet forensics. He was already well skilled in chopping off body parts, as he had been fascinated for years already with it. Practicing on animals or more. He took the half conscious woman and made her walk back out to the boat. Perhaps hit her on the head after they climbed into the boat but before the lake. He took the boat back up near his parking spot. Threw the woman overboard. He threw the woman into the lake because he wanted to hide the DNA evidence from the sexual assault. Perhaps he washed his hands and such in the lake too. Changed shirt. Maybe pants too. He got in his car parked near lake and left. He got back home in time late night, his friends or significant others do not notice anything amiss.