It's a very sad story but he was clearly severely mentally unwell and had phoned 999 himself that day. Had he not taken his own life, my guess is that he might have served most of his sentence in a secure mental hospital rather than a prison. I assume that at some point some more details will be revealed as part the enquiry into the police failures, but at the moment we have no idea what his mental state was - for all we know he might have been in the grip of psychosis or schizophrenia. Without knowing the precise nature of his mental health issues, we just don't know whether this was a case of someone killing their children because he didn't want them taken away, or a case of someone killing their children because he sincerely believed they were incarnations of Satan and and was hearing the voice of God telling him to kill them to save humanity from extinction.
The reporting of this story has been really weird. It was obvious from day 1 who was responsible and they already reported the knife wounds so how is this breaking news today? Just come out and say family annihilation.
There will definitely be legal reasons for this. Reporting of crimes like this is way, way more complicated than you might imagine. There are a lot of restrictions which might apply, and some of those restrictions might be in place to protect other family members.
It's also not a 'typical' family annihilation. The woman who died wasn't the children's mother and she didn't live at the property - she was their aunt and was visiting.
It's unclear at the moment whether they know whether the children's mother is, because they haven't mentioned her at all in any of the coverage, but the police had visited the house in connection with a missing person case before Christmas, so it seems likely that the missing person was her.
Bear in mind that, if they don't know where the mother is, there might be an awful lot that they're not able to say until they've located her.