I've realized with this case that you cant shake your own bias from speculation. So I can only see this case through the lens of a gruesome murder of classmate of mine while in high school, but it makes for a theory.

My guess is a "serial killer" in the sense that not driven to kill by revenge/emotional motivations, but rather an urge to kill. A house with 5-6 girls in it sounds like a certain type of SK's dream. I wonder if he intended to kill everyone, started with M+K and then was surprised that E was home. Killing E + X was more difficult than expected with one or both waking up and E being male. He probably expected to go room by room through a bunch of passed-out drunk girls but then in both rooms he entered there were two people in each bed, which is not as simple as killing someone sleeping alone. It was off-plan and the risks got too high for the final two and he bailed.

In high school, a classmate and his parents were stabbed to death . The FBI was brought in, and after a month or two no suspect was named. Everyone theorized that it was connected to the parents and that was the focus of the FBI. Drugs, owing money, etc. Eventually another classmate was arrested after his father turned him in after finding his bloody shoes. It turned out to be the best friend of the victim. Then everyone theorized about what emotional motivations he might have- secret affair and internalized homophobia or disapproval from the parents. But that wasn't the case at all. The murder kid apparently was a budding serial killer and wanted to kill someone and chose his best friend out of convenience- he knew the house, family's routines, way to get in, etc.

In these Moscow murders, the police have said 'targeted' and everyone starts thinking of emotionally-charged reasons someone would kill four people (Or one person and kill three more as collateral damage). But targeted for someone with no empathy/desire to kill is different than revenge or hatred-motivated targeting. It can be as simple as convenience or a reason that would seem random for us with empathy. Same goes for all of their decisions. For example, I read people assuming the lower level roommates were "spared" or ask "why was the dog spared?" as if the killer must have had "human" emotional capability, ability to discern who deserves to die. Honestly, a psychopath probably would find killing a dog boring. But we tend to look at cases as if the killers have the same emotional motivations we do. And most often, yes, it is an emotionally-motivated kill. But when it's not, as in the case of my high school, it leads to nowhere.

I dont think killing is easy for most, and if you wanted to kill four people easily, you'd probably choose a gun and distance yourself from the act. I think the choice to use a knife is because the killer wanted to really feel the act of killing. Even if you were only intending to kill one person, you know the target lives in a house that size and if there is a chance of collateral damage you'd probably choose a gun because easiest to quickly kill as you flee.

So my theory it is that it is someone in the vicinity or known to them with budding psychopathic tendencies that got their eyes set on that house and wanted multiple kills. Maybe it was someone who has killed before and just got too greedy/ambitious with a house with that many victims- messy crime scene or not observing E was home. But I'm thinking they're young, still getting the hang of it or starting out, and felt safe killing in the area they know but not experienced enough to look out for or by dissuaded by shit like boyfriends or two girls going to sleep in the same bedroom or a dog. I just get a sense of cocky arrogance.