I haven’t followed the girlfriend angle too much. But I had the thought, what if it was a girlfriend or acquaintance at the dorm, and he told her what he had planned for later in the day (Norris Hall.) If she threatened to go to authorities, that would be a reason to shoot her. In that case, the 7 AM shooting would have been the random shooting, and the Norris Hall shooting the planned event.
4th person account here but a friend of a friend of mine has a brother who is a state trooper just north of Blacksburg and he has said that it was a disgruntled boyfriend. No additional info on how killing her turned into killing 30 more people then himself.
Take it for what it’s worth...