Awesome post. Already experiencing life as difficult, he thought he had finally made it out of the valley and up to the top of something - then he's disciplined at school; he has "altercations" with his supervisor; he has disciplinary procedures against him, and is ultimately fired from the TA position that enables him to pay the rent and also will give him a tuition discount. He's possibly not going to get to come back in January. His life is ruined, from his POV and somehow, other people still survive, have fun and graduate - he can't stand it.
I'm not sure he had a psychotic break though. I think he could drive, plan, reasoned his crime out fairly well, perhaps even trained for it. I keep trying to find parallels with other mass murders (or serial killers).
There are aspects about this crime from both categories.
In the US, we've had more than 160 people decide to become mass murderers since Jan 1, 2023, IIRC (I think it's now at 169). It's unlikely any of them were thinking about "making their life better." That's certainly not the motivating force for killers, IMO. And they are functioning so well, they don't seem psychotic to me (TBF, the psychotic people I've seen have all been inside hospitals as they were not capable of functioning in ordinary reality). So maybe "quasi-psychotic" is the term I'm reaching for.
This crime is a little bit of this and a little bit of that (I think of Joe DeAngelo/GSK). He was a policeman, for gosh sakes. How did he "rationally" believe that raping and then raping/killing so many people was making his life better? I mean, maybe he did in some perverse way - in which case, so could the killer in this case. How did the woman who just shot up the school in Nashville make *that* decision? We're not dealing with rational people in either case (and I disagree about Rational Choice Theory, BK's favorite theory according to his professor - although it's interesting in that he did apparently think that theory had value).
Other aspects of this crime are Ted Bundy in rampage phase. Richard Speck comes to mind. All of these were mass murders, and all focused mostly on women. I think DeAngelo started out with hot prowling (with his brother, in Visalia) but shot and killed someone early on in his career (both as a policeman and as a murderer - he shot a local teacher who was trying to protect his daughter after DeAngelo entered the house, having been a Peeping Tom prior and having been seen by the terrified girl outside her window; Dad was prepared with a weapon, but DeAngelo shot him first, IIRC). Wasn't caught for what...30 years or more? DeAngelo escalated over those years and then "retired." There are many theories in psychiatry, anthropology and psychology about why such people retire; DeAngelo has given us absolutely no help in understanding his trajectory.
Ted Bundy had once been a university student. DeAngelo had some community college, IIRC (his wife was pursuing her education and became a lawyer, he supported her while she did that). Richard Speck was on the other end of the educational spectrum with a poor home life and the background of a laborer. Speck appears to be a one time major offender, with minor crimes before that. DeAngelo of course was a hot prowler, I believe Speck was a robber and a burglar (knife wielding). Bundy and DeAngelo of course were married, Speck was not and had a pattern of random violent crimes (he once had a girlfriend, sort of). Edmund Kemper has a very high IQ and no evidence of psychosis (according to Donald Lunde, M.D., the psychiatrist called in to study him; book is called Murder and Madness). Lunde thinks that Kemper was, basically, a psychopath/extreme expression of what we now call Antisocial PD. At the same time, we now know that Antisocial PD seems to have a strong correlation with certain parts of the brain not functioning well - and we don't know if it is inherited entirely or also partly acquired.
IOW, different kinds of people can be the perpetrators of violent, random, seemingly inexplicable crime. Being well educated is not a protectant (I personally collect stories about professors and doctors who kill; and students who kill - grad students in particular; there are few patterns or common diagnoses that I know of). I was once part of a true crime reading group consisting mostly of nurses, and they all wanted to read the doctor murderer stories. Education does not prevent a person from becoming a murderer, nor were all the doctor murderers particularly well organized/good planners (or we wouldn't know about them). Where BK fits into all this is mystifying. I think the phrase "he just up and lost his mind" will also stand in well for "psychosis," but I also think that he might not have lost his mind. His mind might normally be made up of all kinds of horrible plans, notions, impulses, etc., which he himself is interested in studying. I think most of the doctor-murderers were not psychotic, either. Just cracked and then cracked wider open.
Your theory about the mind of the killer (if it's BK) is very interesting. If he had gone after his professor, that would have made way more sense - but you do give a reason for why he goes after these four promising and happy young people who had social lives and friends.
If it's him. IMO.