Setting the serial killer speculation aside for the moment, and assuming that all four victims were killed by the same individual, what are the chances that this was the killer's first time taking a human life? I've heard at least one respected profiler hypothesize that the killer had killed animals previously (she was notably less confident that he had killed a human/humans before).
Acknowledging there's a lot we don't know -- how messy or tidy the crime scene was, whether/to what extent the killer was wounded during the attacks (e.g., from knife slippage), whether the killer knew how many people were in the house and which rooms they were in -- I think it is staggeringly improbable that someone who had never stabbed a human before would have the composure/confidence to sneak into a home occupied by multiple college students, kill four of them quickly and quietly enough to avoid being detected by the housemates or neighbors, and then successfully flee the scene and disappear into the night. To execute a mass stabbing under those conditions, I think the killer would need either (a) confidence gleaned from the experience of having killed before, (b) chemical courage that would remove inhibitions/stress during the performance of the killings, or (c) rage so consuming that he (temporarily) did not care about being caught.
I get that the victims were sleeping and had been drinking and therefore were in a vulnerable position. But the killer assumed substantial risk in order to carry out a mass murder in that setting. In my opinion, there's a good chance he had killed before.