I was thinking about scenarios that included decapitation, and IMO, some of the categories that it might fall under:
Ease of transport or temporary concealment (like in boat, as you mentioned).
Failed complete dismemberment: There's the "ran out of time" possibility, the "overwhelmed by the gore" hypothesis, and the "misunderstanding the plan" scenario, which would necessarily mean there was more than one perp involved in the murders. IMO
IMO, Injury inflicted that would leave evidence tying the perp to the crime: bite marks/signet ring impression/shoe print.
Removing evidence from the scene
If a sexual assault was perpetrated on Mr S by way of his mouth (yikes! sorry for having to be so graphic), decapitation would remove all evidence (dna) that would lead to the murderer(s) IMO
If the depraved perp bit Mr S on/around his face or head, they may have worried about leaving an impression which may link to them.
I know that a gun has been bandied about as a possible murder weapon, but what if the weapon used to inflict injury/death on Mr S was an unconventional weapon?
A hunting/archery arrow: couldn't remove it from head
An object like a bowling trophy, or other unique object that could be traced back to the perp(s). Hey, if I'm going to speculate, might as well push the envelope, LOL and MOO