Parents Guilty of Torturing Disabled Child

I couldn't let this story pass without mentioning it, seeing as it fits in with some of the topics that I've been discussing recently. According to the BBC, a couple have been jailed for over 10 years each, after being found guilty today of abusing their 4-year old, disabled daughter:

The little girl had boiling water poured over her hands, had huge clumps of hair ripped from her head, was kicked so hard in the groin she suffered internal injuries and was locked naked in the toilet each night. She was also forced to eat her own faeces.

She was due to be fostered to a family but a report from Westminster Council in January last year recommended the girl be placed back into the care of Duncan and Harte. Although it is understood up to 20 visits were made to the family home by social services and other care professionals, no abuse was suspected.

So this little girl was let down by her parents and society, but at least she's not dead. Other children have not fared so well. What motivated these humans to treat their child in such a nasty and vindictive way, I'd like to know. Whatever the reason - if any - their behaviour is inexcusable and I can't help but feel that they should not have reproduced in the first place...