Yes, you are singing my song fwiw. Robbery doesnt appear to have been the motive for sure.
That is quite a leap to go from robbery to slaughter. 
I thought maybe the reason there was someone in the home that didnt raise alarms is because there was a certain comfort with it from having been in it before. I genuinely believe that if it was someone targeting little angel, they believed this was an empty house other than her. If the weapon was from inside of the home, the intruder was carrying it when he encountered Leila.. dont you think?
BBM, and well-put: botched robbery doesn't usually seem to lead to this sort of raw slaughter.
Not sure if killer assumed Leila was alone when she was killed. (Most 8 year olds aren't home alone, are they?) If killer was an intruder/burglar, then seems these would be the scenarios:
1) intruder is lying in wait, sees family leave house, goes in. Likely through a doorway, as by several accts, people didn't lock their doors/windows in this neighborhood. But maybe through a window, possibly even a bedroom one.
But:
• If Leila was killed in her bedroom (not ever stated, but I'm leaning that way due to the fact that a twin mattress was hauled out for evidence), then this doesn't make sense to me. Why was there no account of screaming by Leila, either before or during attack? If intruder saw her, she at some point saw him. Was her mouth taped? Did a burglar come armed with duct tape in case he wanted to commit a brutal murder? Did he sneak up on her unawares while she was sleeping? (at noon?)
•Seems an intruder intent on burglary would likely hit the TV or electronics room, first (main living area) if he thought the house was empty. My sister was burgled and they load up the TVs and electronics near the point-of-entry door for easier loading into a getaway vehicle. But unless the boy was also in his bedroom at 12 noon, why wouldn't he have seen or heard him prior to or during Leila's murder? The house is not that big.
OR, 2) Killer did not lie in wait and/or did not know family had gone:
•If the killer were a more polished intruder/burglar, OR had not seen the family drive off to the LL game, he might have first scoped out the house to make double-sure it was empty. But if he encountered Leila along the way, wouldn't she have screamed? If not upon seeing him, at least while she's being stabbed 21 times? Again, why no accounts of screaming?
