This post ^^ is the most important post of all. This is where all the answers to this tragedy lie.
*If there was only 1 killer, it seems improbable that the removal of Mr. Dermond's head was to get rid of evidence. (Ballistic evidence, specifically).
*1 killer would most likely kill both victims with the same weapon, in the same way.
I don't see Mr. Dermond being shot and the perp throwing the gun away and then killing Mrs. Dermond via bludgeoning.
As far as decapitating Mr D because of potential DNA evidence, that doesn't fit either, IMO. Mrs. D would probably have the same DNA evidence and the killer(s) could have just dumped him in the lake as well.
I also agree with Oceanblueeyes regarding everything she head about it not being a professional hit. Nothing in this case fits with the MO of any professional 'hitters' that I have ever seen.
This case is making me :scared::banghead::scared: