There are psychiatric disorders where a person believes themselves to be in a closer relationship with someone than they really are.
Sometimes they are stalkers, but sometimes not. They may see themselves as the "next boyfriend" or the "true boyfriend" (or girlfriend) of someone who barely knows they exist or is simply trying to be socially polite with them. Usually, this does not result in tragedies like this one, but if someone who had been hanging out at the house/attending parties had the view that one of the girls was "his" (or about to be "his") and that girl was rebuffing him and the roommates were all talking about banning him from the house or did ban him, it's possible this perceived slight was a trigger.
... Just as if he were a real boyfriend, because in his mind, he is the right person to be the boyfriend or is almost the boyfriend. The people who knew about this fraught relationship were the girl and her closest friends (perhaps just 3 other people). If he knew that her roommates were talking smack about him (as he might put it), that could cause exceeding rage in the right (deranged) person.
Does anyone know any data on how often a mass murder perp returns to the scene of the crime?
I don't think there's any large statistical analysis or even a recent literature review on this topic, so if someone finds something, please post. We just know that it does happen.
In the scenario I suggest above, I would predict the perp would return. I just read a case last night where the murderer was so deeply trusted by the family of the deceased that he was a pall bearer (and arrested just a few days after the funeral of his victim - and eventually convicted). This person could be quite close to this circle of friends, to the point that it would be weird if they didn't offer emotional support/visit the remaining roommates, etc.
