I'm not convinced that the killer didn't come home -with- E&X. Someone who claimed they needed a place to crash for the night. Maybe said they'd had an argument with their girlfriend, or had locked themself out, or perhaps got tossed out of the frat?The more I think about this, the more clearly I see the possibilities:
1) The killer was present with either K&M or X&E prior to the murders, most likely at the food truck or the frat party. The killer would have been present at the moment of the victim's departure. Otherwise, how would he know when they went home? The killer would also need to know the victims well enough to know where they lived.
If E&X invited the person to sleep on the couch, they'd be in the house, know where everyone was sleeping, and know when things got quiet and everyone went to sleep. If they came in after the 1st floor roommates went to bed, they probably wouldn't even know anyone was down there.