I think Sills stated that RD was deceased before he was beheaded. I would suspect the coroner could tell.
Yes, Canaille, the Putnam County Coroner/ME confirmed that the decapitation of RD was post mortem..
Imo, there were only three likely reasons for the decapitation and removal of the head from the scene;
Forensics (eliminate possible touch dna or body fluids of perp)
Ballistics
Souvenir-to later relive the crime