Can someone help me with a potential motive for multiple murderers to kill these four, yet leave two potential witnesses alive? I would expect a group, who thoughts things out ahead of time, to be prepared enough to make sure they left no potential witnesses. Someone would have to be the ringleader and convince the others to carry out this plan. Multiple murderers also makes mistakes more likely. Even with a murder for hire, I think it's unlikely that this would be a group of well-prepared attackers. Normal people wouldn't know where to find the kind of assassin TV has trained us to think of, nor would they have the money to pay them. And a real assassin wouldn't choose knives as their weapon. Too messy, which increases the risk to them. Makes a clean get away more difficult.
The more I've thought about it, the less I'm inclined to believe sudden rage motivated the killer/s. We've heard that LE believe the crime was targeted, but we haven't seen much about what kind of attack it was. Rageful overkill, or what. In general, rage makes people less organized and less controlled. Not quiet. I don't see someone like that sneaking into the house. Or laying in wait. Rage demands action. I'm also doubtful of the ability to maintain rage after killing two and going to the second area. I would think just the time it takes to change floors would be long enough to start thinking "OMG, what have I done?" Would someone having those thoughts be able to even continue in the second area? I don't know.
It's possible E was not as much of a problem as he might first appear. They've been partying until the wee hours and have only been asleep a short while, and it's the time of night where the body is at lowest ebb. I haven't read anything about E's personality, skills or habits, so I have no idea whether he would even attempt to stand his ground against an armed intruder or try to escape the room and get help. The former would be heroic, the latter totally human and understandable.
Hatred killings are the ones I find most difficult to understand. The mass shooters that immediately spring to mind don't seem to care whether they lived at the end of things or not. They just want to kill/punish as many of the people they hate as they can. Although I did read of several stabbing mass murders, I haven't read any detailed information about those cases. It does feel like these killers didn't want to be caught, and that he/they either killed everyone they came to kill, or were sufficiently disorganized that they didn't make sure they had found everyone in the house. And I'm back to whether the locked doors would be a real obstacle to a determined hate killer.
I would love to think the investigation will progress a lot after the holidays, and it certainly may. But I wonder if the killer/s remained in the area at all, or will return to the area. A professional killer (although I don't agree with that scenario right now) would not stay around and wait to be caught. They have no reason to do that, and their best defense is to disappear for good. A rage killer might be shocked by their own actions and possibly seem different to friends and family. Agitated, worried, brooding. Perhaps even nightmares. I can't imagine such a person would go back to Idaho and wait for the police to figure things out. The hate mass killer might feel quite justified with their actions. Would they welcome the eventual chance to tell why they did it? I don't know. Either way, the police may end up having to extradite someone from another state. IMHO.