I don't really doubt that the other roommates didn't hear anything if what LE says that EACH victim was asleep when they were killed is true. The killer may have quietly gone room by room putting a pillow over the victim's face and stabbing them.
Because of this, I am having a hard time thinking about who the killer could be. If it were truly "targeted" and "personal," they could have stopped after their target. It makes no sense for the murder to be targeted AND kill 2-3 other people in their sleep. That's like dedicated thrill kill or serial killer behavior - where the goal is the killing, not the elimination of a person out of an emotional reasons (love, hate, anger, betrayal).