I also unfortunately feel they don't know who did this yet. According to the interview with Kaylee's parents, they have lots of evidence, so hopefully that leads them somewhere. If the crime scene was a mess (also stated in the interview with Kaylee's parents), that's a lot of evidence to go through and parse out. Hopefully they get something that points them in a direction or someone comes forward with surveillance footage that helps.
I am also not 100% convinced it was someone known well by the victims. It can still be targeted and be someone unknown to them. Like I would describe the Jayme Closs case to be targeted but the family had no idea who he even was. Were the four victims ever somewhere together (likely they were at one point in time together in public) where they inadvertently made someone else mad? Maybe they had no idea they even did, but this person was ticked off enough to follow them home one day. Or perhaps someone who lives nearby who is mad about party noise coming from the home. Yes, two roommates were spared, but could it be as simple as, their bedroom doors were locked and the other four were not? I don't know, just thinking out loud.