I'm thinking the same thing.
They knew she was at home alone with her son and took that opportunity to seek entry into the house (whether they knew her personally and entered under the ruse of some type of business or if they knew enough about her--her schedule, demeanor, etc.--but she might not have known them. Maybe they posed as one of those vacuum cleaner salesmen that the media reported had been spotted in the area that week). The fact that neighbors distinctly recall a prowler in the neighborhood, someone banging loudly on a door at some point, and "aggressive" vacuum cleaner salesmen in the area... seems like a big piece of the puzzle to me. It would appear that someone was scoping out the neighborhood. Maybe they were looking for an easy target and felt like they found one.
After they gained entry, maybe they threatened to hurt the son and forced Amy to call her husband to convince him to come home. Maybe there was no go-kart accident and that was just an excuse to get him home or an excuse to give the housekeeper as a reason for leaving earlier than planned.
Maybe Amy attempted to flee w/ her son initially... in the Porsche (which would explain the speeding or erratic behavior of the car that was seen leaving the house by a neighbor on Wednesday)... but the perp jumped in the car and/or ran after her and attacked her... which would leave his DNA behind and might be why they chose to torch the car the next day.
So many possibilities!
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