The 6 of them came home from various activities throughout town. They came home to go to bed, IMO. The downstairs women may or may not have come home together - or merely at around the same time (1 am).
The four victims come home later, around 2:30 am IIRC, but not with the intent to party - the intent was to go to bed.
While this house was a "party house," it appears they specialized in "pre-Party" activities (before games or before bigger parties at frat and sorority houses). I'm not saying that they didn't have large parties that go all evening, just typing what I remember from a local article (in the media thread).
Further, almost no one starts to drink more/party at 2-3 am. That's time to wind down, rehydrate, and rest.
K and M were together all evening and went up to bed together, as friends do. X and E were together most of the evening, and then went to bed, as a couple does. The two women downstairs may have gone out together - not sure we know - but they slept in separate rooms, as they usually did.
Everyone behaved normally, IMO.