So far, the PCA indicates only one single source of DNA on the sheath snap. We don't know about the rest of the sheath.
He failed to study Rational Choice Theory thoroughly enough.
Touch DNA can last varying amounts of time, depending on use of the item and many other variables. I'm not as sure as you are that one can tell how long the DNA has been there.
I think he pinned the victims down by kneeling on them (legs) and used the knife in its regular manner, for accuracy and to avoid blood spurt/spatter onto himself. He likely still got bloody but would have had way less blood on him with this method.
This is one part I think he thought out and may have practiced. He probably flew across the room as rapidly as possible to surprise Ethan, knowing that if he could put his own weight on Ethan's lower body, Ethan had no chance. I think we'll find that each victim had 3-4 major wounds, not 10 or 20 or more.
I think BK had been role playing inside games with similar weapons for a long time, and had spent quite a bit of time thinking about how best to murder people in their sleep.
This is my supposition. I do think he planned the core part of his murders, which is the manner in which he used the knife, his blood-reduction use of bedding and mattresses, and the actual organ systems he targeted, using the Marine-style hold on the knife.
As far as we know at this time, only one shoe had a faint amount of blood by the time he walked past Dylan's door.
The autopsies will tell the story, though, if we ever get to see the reports (I assume we will, some day).
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