One of two scenarios
1) That this being the first time Mrs McCann has been left with the children instead of a nanny looking after them, she snapped and lashed out
or
2) (probably more the situation) that Maddie was killed acidentally, either by overdose or hurt herself when left on their own one of the nights on the holiday.
If your child was kidnapped would you really go back to the crowd at the bar leaving the other two on their own in the apartment?
Is all this information factual or has it come from the newspapers?
I havent read up on the case much just seen bits on the tv, but going on their body language and their own reactions i personally do believe they are innocent.
Who knows-I maybe wrong you,may be wrong and we may never find out!