I'm going to add in a controversial bit of analysis (hoping that at least one woman who is thinking "he'd never hurt me" will benefit.
PF had two parallel traits: the ability to transform rage into a killing scheme (not just impulse) AND
the ability to amp up into full-on "hunter mode" when the prey tried to escape.
I have seen this type in action (angry and abusive, very violent - but then absolutely murderous if defied/the prey tries to escape).
The first action (premeditation, bringing a bat, all of that) amped him up enough to administer the first blows. But he hesitated long enough that she could beg for her life in two small words. She may have tried to get up and lunge toward the living room, he amped up the attack. I believe she fought for her life as best she could. And that enabled him to be even more brutal, as he knew it would. She, on the other hand, had no pre-planning and reacted as best she could. He liked the unfairness of it. It was the basis of his MO. "Pick on someone who is unsuspecting and relatively helpless; convince yourself they really need to die; get more agitated if they try to escape/evade their own death."
This is a domineering, controlling, impulsive sociopath.