Something I haven't seen anyone suggest yet, is the possibility that the killer didn't enter just before the murder, but perhaps entered much earlier, (maybe not even through the sliding doors) and hid in Kaylee's or MM's closet (or under the bed, or whatever) for a while and waited for his victim(s) to come home and go to bed before attacking.
It's been 40 years since I was in college, but I remember back then, in any similar situation, with 6 students living in one house, there would be a constant stream of visitors in and out...especially on a weekend...it might be hard to keep track of all of them.