It seems to me that two main questions keep popping up -- and we've all taken shots at answering them, usually considering them separately.
(1) Why the decapitation? to easily remove evidence (bullet); to send a message; psychopath's thrill-kill fantasy; organized crime/drug cartel signature ...??? (Others?)
(2) Why leave Mr. Dermond there but remove Mrs. Dermond (living or deceased)? to remove evidence (bullet lodged in chest, for example); for possible ransom/demands; to (try to) get information/access accounts; psychopath's thrill-kill/possibly sexual fantasy; some kind of emotional attachment/hesitation on the part of the killer/s; couple was seized elsewhere (even just outside, on the grounds) and Mrs. Dermond was never brought back to the residence; just took her as "insurance" (hostage) in case killer/s was interrupted while leaving the scene, etc. ...? (Others?)
Would it help if we consider these two questions specifically together?