The only thing that stands out to me is that he's not afraid for his own safety. I could imagine he's blocking the tragedy out of his mind. If he hasn't experience death before, he may imagine it's not permanent; or its permanence may not register with him emotionally.
I find the notion of an intruder able to do what he said, take her quietly to the basement and murder her, very scary on a visceral level. He should feel safe in his house and room, but if an intruder could get in and kill someone without anyone hearing it shatters that notion.
Kids are normally afraid various monsters and boogymen. I would expect some kind of anxiety or odd behavior of some sort centered around the threat of an intruder.
This makes me think he knows who did it. I don't know if it was him. But I don't think slept through the whole thing. The same goes for his parents. I get freaked out when my kids (5 and 3) are playing the back yard and lose sight of them. If a kidnapper or murderer even attempted to get into my house, I would not let the kids out of my sight. If the kidnapper were on the loose, I literally would make my child nap in front of me, not in his room. I wouldn't turn my back.
No one in this family shows a normal sense of paranoia associated with an unknown criminal. That makes me think they all know who did it. It could have been any one of them or some family friend, but they all know who it was.