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.
OR
2) The killer watched and waited to strike. He would have to know the occupancy of the house to know when everyone was home. How else could he be sure that other roommates or friends weren't going to come home in the middle of the act? The killer would live within sight and walking distance of the house.
I don't see any other likely possibility, other than some disturbed mind picked a random house to attack and got lucky not to get caught (yet).
But assuming the killer didn't just pick a random house, which should be known from the forensics, then logic says that the killer must have known when the victims would be arriving home. Therefore the killer was either with the victims when they departed to go home, or the killer was already watching the home (either from inside his own home or from inside the victim's home). I don't think he was spending hours in the frigid dark weather hiding in trees, waiting for the victims to arrive. He would have no clue if they were coming home at 1, 2, 3, 4, or even 5 am. Or even not at all! He also would have turned his phone off, so he would be waiting for hours in the cold dark with nothing to do. Every minute he spent waiting was another minute he risked being seen. I guess it's possible, but I don't think it's likely. He also could have been in their house already, but that is very risky.
Look at the people who were present with K&M and X&E, and look at every single person who lives within sight distance of the house, and I bet you will find the killer.