I can't help but think the girls' playful and carefree nature, perhaps flirty nature, played a role in why they were targeted. (Yes, I do think MM and KG were the targets.) A sexually frustrated man, older than college age, with some military/weaponry knowledge, who felt ignored or rejected and decided to get revenge. He staked out the house for days or even weeks, and got a feel for their routines before he made his move. He would have killed everyone in the home, but started getting nervous (maybe heard something) and left without going to the first level. We don't know if the survivors heard anything, or got up from bed and made noise (flushed a toilet, turned on a faucet, opened a drawer) while the killer was there.