I agree. It's easy to become slightly desensitised because unfortunately so many children have been victims of abuse in high-profile cases but the level of violence in this case used against this poor girl is extreme and horrific by any standards. Just gratuitous violence and torture from start to finish. In some of the other cases, you could perhaps, if not excuse, at least see how the situation might develop due to poor living conditions, mental health issues, parental stress, inability to cope under the pressures of life, immature emotional regulation, loss of temper etc., even while your heart was breaking for the children involved. But in this case, no. It's difficult to see any explanation aside from the most horrific sadism.
I do think the neighbours are culpable to some degree, although not perhaps as much as some other people on this thread think. This case is a reminder that if you have doubts about a child's welfare, always make a report even if you think your doubts might turn out to be unfounded. It's important to remember that the neighbour's didn't see the abuse itself, they heard it, yes, but then they saw children who looked well cared for and they had the father apologising for the noise. So it may not have been clear to them exactly what was going on, especially if they weren't very bright or didn't have experience of children to distinguish between normal child noise and behaviour and signs of abuse.