And we cannot forget that the killer had as many as 9 hours (3am-12pm) to clean up / dispose of any evidence.I definitely think it’s true that if the killer was not wearing some sort of PPE/hazmat suit that he had to have had at least some blood splatter on him. Even if it were just one stab wound, with him slowly removing the knife, I believe he’d be lucky to not have any blood on him at all.
The fact that we know it was many stab wounds on four victims, likely in the dark and likely somewhat in a hurry, with all that we know it just seems that there must have been blood spatter. Especially since (as you said) the walls had blood on them.
I've felt from the start that the perp wore a hazmat suit, booties, and gloves. I think after committing the crime, he stripped out of the soiled outer layers, and stuffed everything into a backpack or gym bag. He then probably left the scene on foot (not by vehicle), likely undetected by bloody shoeprints, and went back home to clean up, and possibly left the area in the following days to properly dispose of the bloody outer layers he was wearing during the crime.
There certainly would be nothing strange about anyone seeing a guy at that time of the morning walking around that area so close to a college campus carrying a backpack or gym bag. And it was cold on 11/13/22 at 3:00 a.m. in Moscow, ID (28 F), so if he was wearing a hooded coat and/or a balaclava facemask, no one would find that odd, either.
{{ THIS IS ALL MY OWN OPINION }}