My impression is that when someone is killed because their murderer had a fit of uncontrollable rage they are spectacularly beaten-up and their intention is to vent anger, not necessarily to kill. There is a trigger that makes them 'snap' and they lash out. To this end, if Sion Jenkins attacked Billy-Jo because she answered him back etc I would have expected him to punch her, or shoulder-charge her etc. If as his ex-wife claims (and we have no reason to doubt her) her had hit them all before, without the outside world knowing it, he must have been adept at controlling his rage and only hitting them enough to scare them, and in a manner that didn't leave visible marks. the purpose of the outbursts, if you read up on domestic violence, is to control the victim.
Coupled with the very small window of opportunity to do the murder, if it was Sion Jenkins I'd have thought it must surely have been premeditated. Likewise if it was a random stranger they must have been observing B-J so it will have been premeditated.
It is possible Sion Jenkins had a reason unknown to us for killing B-J (we'd have to speculate). He would have been taking a hell of a risk with his daughters there though. I believe his daughters testified that they weren't actually allowed inside the house during the brief period they were back, they stayed at the front with the car. This possibly opens the window of opportunity and she may already have been dead. It seems hard to imagine that much violence could be inflicted close to someone's head and the perpetrator not get visible blood on them. However the blood on his clothing wasn't visible to the naked eye. If he had killed B-J earlier he would have had an opportunity to change/wash/dispose of his clothes.