I don't know why you think the parents would have had to be monsters to be involved in some way. You also seem to think that you can judge a persons character just by looking at them. I don't think it works like that.
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?