This is crazy. When I heard that they "always" left the door open, for some reason I thought they meant that they would leave it unlocked as long as they were home so that they wouldn't have to keep going to the door to let people in. But really it is literally unlocked all of the time, even when other roommates go out of town?!? You know when I was in college I would sometimes accidentally leave my keys in the door or had crazy careless roommates, but this is really odd that they had no concern and remained oblivious to the crimes that had already happened to other people in that very same neighborhood.
They weren't even worried about any of their belongings in the kitchen and living room areas being stolen or vandalized? In Mark Fuhrman's video it looks like a very nice place, they even had a full size washer and dryer laundry room. That is lovely that they locked their individual doors, but it is beyond me how they didn't realize that something else could happen in another area of the house. They treated this house like it was a dorm, everyone was only responsible for protecting their own individual room. Why even rent a house then?
I agree with someone else who said that this completely opens up the possibility now that someone was already waiting in that house, especially someone who may have seen the male roommates packed their cars and leave earlier. It could even be an acquaintance of the male roommate(s), he would know about the door lock situation and maybe even know whether KT slept through noise.