unless the killer was in the house before the doors were locked or (the hypothetical security system was armed)
against better judgement here's some speculation. the killer was hiding out in the parents bedroom closet, everyone goes to sleep, the father was done in first, stabbed as he slept, then the mother as she was awakened. the son heard the commotion came to see what's up and got it in the hall with the dagger, the daughter was the last to awake, as stated the killer wasn't expecting four occupants and was on the way downstairs and/or out the door when the daughter spoke with the mother and fled to the neighbors.
I was in the neighborhood Thursday, and it seems fairly secluded, even being right along 501 and all things considered, doubtful that this was a random act. I'm sure motives like insurance and such were considered, without more information it's pretty tough guessin.
one would think that a blood trail would be found outside the premises leading to a get away vehicle or at least a direction of flight, after perpetrating such an act a vehicle would be the most convenient way out, I'm surprised that the daughter or neighbor didn't notice a vehicle leaving the scene. I doubt many vehicles were cruising the loop at that place and time.

I was in the neighborhood Thursday, and it seems fairly secluded, even being right along 501 and all things considered, doubtful that this was a random act. I'm sure motives like insurance and such were considered, without more information it's pretty tough guessin.