I don't think the motive is bad. A bit obvious, perhaps. But a classic for a reason. But sometimes I wonder how it would have been if they chose for there to be no motive, it's just completely random, and not a couple of their kids in on it.
sharespoiler alert
well, it certainly would have been a much different movie, some like The Strangers maybe.
Part of what makes this movie so good, and hurt so much on an emotional level is the betrayal. Felix urging Aimee to run out the door at full speed. Zee staring at everything wide eyed but not really surprised. And of course, the full reveal at the end.