On the other hand .......
Double murder requires an endless list of requirements that defy logic
~~Murderer would require access to the home ahead of time , unknown , and unseen by anyone
~~Murderer would need to intimately know the home , security , or lack of security , cameras , lack of cameras etc.
~~Murderer would require advance knowledge of the Sherman's travels and plans for that evening
~~Murderer would require advance knowledge of staff , housekeepers , plant-waterers , real estate showings , where the belts are kept etc.
~~Murderer would be vulnerable to exposure by having to lurk and hide for the several hours between when Honey and Barry arrived home , we are told they were both wearing their original daytime clothing and jackets.
If indeed it was an outsider-enemy , and if indeed it was a passionate-murderer who desired to "leave a message" he failed because no clear message can be gained by the staging by the pool.
Rather , three possible "messages" emerge , suicide-suicide , murder-suicide , murder-murder , or something else. A lot of wasted effort to create a message that does not work.
If "leaving a message" was not a requirement of the murderer , two bullets in a parking lot would have accomplished it. The Sherman's were not cloaked with security during their downtown travels and life . A bit of stalking is all that would be required.