It is manslaughter because there has to be an intent to kill for it to be murder. Sadly, sheer b.....y stupidity and pig ignorance is not a crime. People like this man do not think things through they just see the whole process from their own point of view. He wanted custody of his children by his mistress and he thought no further than that. He thought if he could incriminate her in the arson and save his children he would be a winner all the way, get a bigger better house, get custody, be a hero the lot.
If he had even begun to think it through he would have realised that with modern forensic methods it would be very difficult for him to disguise the fact that he was involved. Not realising that forensic examination can identify individual brands of petrol was one of his many mistakes, talking about it not realising the place where they were staying might be bugged was another.
Sadly stupidity got its reward. His children died, his surviving children stay with their mother and hopefully, given his past record he will now be given a long prison sentence. I understand there is honour among criminals and killing your own children like this puts you outside this code of honour. His time in prison may not be pleasant.