For me the killer most likely knew the victims. This is a very brazen attack, going after 4 people in their own home (6 if we include the two women who survived). Very few serial killers actually take such risks because coming unprepared in such a circumstance might as well result in your own death, not your victims. Especially given that apparently he was only armed with a knife. This leads me to believe that the killer obviously knew the layout of the house and the people there and he was feeling confident that he could take them. And most likely he stalked them during the night until they fell asleep. He is either a friend or a recent contact of them or at the very least someone who has prowled and prepared.
Killers like to feel comfortable in their environment and minimize the threat to their own life at a minimum. And obviously this guy took an incredible risk. Again, I just cannot fathom a random guy going in a house he knows nothing of, to stumble into 4 people and kill them without ever raising a sound and then sneak quietly into the night without nobody noticing. That seems very unrealistic to me since this killer obviously knew what he was doing and where to find the victims. The human mind doesn't like "surprises".
He also doesn't seem very experienced in murdering considering there was a much easier way to kill his victims and at the same time minimize the risk for himself, but he instead stabbed them. This would either indicate rage, a personal attack or someone who is not exactly sure how to kill. Either way I think he is someone they know or at least he knows the victims. The fact that he also ignored the other two girls, whether he didn't notice them or on purpose, also speaks volumes.