The lack of a sign of a break-in to the property is what really strikes me (as well as all of the other pieces which don't add up).
From what I can gather (although there is so much speculation it's hard to tell what is the truth), the house in question was the "social" house which had people coming and going all hours. It was where friends would meet up, hang out, have parties and generally live the typical student lifestyle. This house, more than others, was apparently considered a social hub by many.
It therefore doesn't seem unlikely that the door was left unlocked, either if it was forgotten or they may have anticipated someone else arriving at some point later on or in the morning so intentionally left it unlocked.
Purely speculating here but if the occupants of a house which has multiple people coming and going had become lapse around the locking of the door (to save having to constantly let friends in and out), it may just not have been locked. I suppose up until now, there may not have been any concerns regarding someone with bad intentions entering the house so it may not have crossed their minds to lock the door each and every evening especially if they'd been drinking. I can't blame them in the slightest - I live in quite different circumstances to them but consider my house to be in a very safe location and therefore I and quite often leave my front door locked (although not at night). You don't expect anything like this to happen, no one does, they can't have imagined this every happening to them.
Another speculation my friend had is that if this was the social house with lots of friends coming and going, more than one additional key to the property could have been cut, which could then have been shared/lost/stolen/copied meaning it fell into the hands of the person who committed this awful act. I also question, if the door was unlocked, whether the perpetrator knew that this might have been the case and if he did know, how? Did he just turn up and hope there would be easy entry to the house? Did he plan to break in somehow if not?
The whole case is an absolute mystery with so many unanswered questions and large parts which simply don't make sense. The lack of many solid facts from the authorities has also meant that large amounts of false information have been circulated online. I will be keeping a close eye on this case, it's a very, very sad one.