The McCanns aren't likeable. But the fact is, it could have happened to any of the children at that complex - maybe other complexes too - because the McCanns weren't the only ones leaving their children at night. As other people have pointed out, it wasn't much different to the way numerous parents in the 70s and 80s, left their kids in chalets at Butlins, and relied on 'Baby crying in Chalet No.....'. For a while, it was perfectly acceptable, though obviously to everyone now, totally wrong. Pre-Madeleine, it was easy for parents to feel lulled into a false sense of security, and believe their kids were perfectly safe. Most of the time they were, at least safe from predators, but not safe from fire, or vomiting, or other reasons why kids shouldn't be left alone, or at least out of earshot.
I don't think for one minute that the McCann's are guilty of anything other than 'unintentional neglect'. They have to live with that. It's hard to feel sorry for them, because a child is missing, possibly dead, but it must be bloody awful to suffer the loss of your little girl and also see yourself vilified around the world and to have to see all the accusations that they killed her.
Even master criminals struggle to conceal bodies - I'm not sure how the McCanns would have done it in such a short space of time, and so successfully. I also find it hard to believe that they would have fought to keep the case open for so long and risk being found out if they were guilty.