Of course they would have to be monsters. We are talking about (HYPOTHETICALY SPEAKING) one parent discovering the corpse of their eldest child and rather than doing what any normal person would do and alert the necessary authorities, call the police, ambulance etc, instead went and hatched a plan with the other parent (a monster for agreeing to this plan in the first place, when they should have been ripped to pieces with grief), a plan which involved finding a hiding place or burial spot for the corpse, thus denying the child a proper burial, and the child's extended family the opportunity to grieve properly, and ALL to save their own skins. Not only that, but these monsters then went on to set up a million pound fund to swindle pocket money from ickle children, and pensions from impoverished OAPS. They then spend the next 10 years carrying off this crime without the merest hint of a crack in their armour, ALL TO SAVE THEIR OWN SKINS.
They would HAVE to be monsters, no question at all about it. The question is, do you believe the McCanns are these monsters?
You missed the "best" bit, though. (You didn't, actually. You missed the worst).
Kate has just made the most devastating discovery of her life, the body of her loved and cherished daughter.
Gerry is oblivious and unaware in the restaurant.
Kate saunters back to the restaurant, imparts devastating news to Gerry and together they hatch a plan (without anyone else noticing).
Kate leaves the restaurant a second time, returns to the restaurant a second time; Gerry leaves the restaurant (without anyone else noticing); saunters back to the apartment, scoops the remains of Madeleine, saunters down to the beach past the noses of the Smiths and deposits the remains of Madeleine; saunters back to the restaurant; whereupon Kate takes her leave, only to return to raise a fake 'alarm'. And all hell breaks loose from there on.
It's easy when you know how ....