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Methodical.....planned then?
JMO. Maybe not planned in advance, but the person is perfectionistic and rolled the plan in his head, many times over. To me, it rules out impulsivity. The person thought of the best day, time, entry, retreat, weather. He tried not to leave traces. It is possible that he minimized any chance of being linked to the victims. He probably operated within a very tight timeframe of an alibi. I imagine that it was not the situation when he killed intended victims and then accidentally ran into someone else so he had to kill those people, too. Rather, he checked one floor, room-by-room, till he found one inhabited and killed its occupants. Then he went up/down another floor, and checked room by room, till he found the victims there. Maybe he checked the basement, found it closed, then left. I wonder if the word “economical” can be used for him. He probably didn’t make much mess outside of the rooms. Maybe he tried to step in the same footsteps on his way back, or left exactly the same way. Probably, he worked fast. One can’t tell from this description if either of the victims were targeted, or, indeed, he had been to the house before these people moved in and knew the general layout. Or perhaps he simply studied the house on Zelle; then he should have good visio-spatial orientation.
It is hard to tell whether the person was obsessed with killing, or obsessed with a certain victim/s. It almost rules out being obsessed with two victims, unless the people were so joined at the hip that two became one. (It doesn’t seem to be the case. X@E were together for less than a year. K@M were close, but in different sororities. I can imagine the situation when he was following one victim, and when the victim moved, say, from a sorority to a house, he changed the place).
As I am typing it, I wonder if this scenario allows for several killers, or a murder prompted by emotions. Neither seems to be the case. The day could have been somewhat triggered by a quarrel, but the plan had been brewing in someone’s head for a long time. Likewise, I can’t imagine two killers in the house.
JMO. Maybe this is what an organized killer is.