Bit of a stretch to think 2 well educated parents left their 3 children, all under the age of 4, night after night in an unlocked holiday apartment but it happened!
No one knows the motive for the abduction. It is completely wrong to assume it was sexual.
If it wasn't sexual or pedo related, one can only surmise what the Tranmers were thinking at the time.
On the drive back to their hotel. Carole says "hey Chris I'm really worried about that poor little girl in the apartment beneath auntie Pam's. What with all her crying going unattended and that weird guy leaving the apartment"
Chris says "should we go back and see if she's crying and been neglected, or should we notify the Police and report a case of child neglect"
"Let's go back if it gets late, we can always stay at auntie Pam's for the night"
The Tranmers park the car outside the apartment, notice all the comings and goings and think something isn't right here, and make a rash decision to 'rescue' the girl.
Then the proverbial hits the fan, and they don't know what to do with Maddie so they off load her somewhere to be raised by someone else.
Rescuing a child and then killing her doesn't add up.
Only a pedo would kill the child because a child is thought of being a disposable item after gratification.