Protect as in protect society as a whole in an emergency such as a storm or earthquake, but not individually unless they happen to see it in progress.An example would be a swat team in a hostage situation. The crime has already been commited, now they need to end it but it is not their obligation to prevent it unless they knew it was going to happen. In the story above did the police know the guy would kill his kids, the answer is no.
East, as I recall, the purpose of the restraining order was because he was repeatedly threatening to kill the children's mother. I don't think it's much of a stretch. Besides which, the restraining order should have kept him from taking the children in the first place.