Ab.so.lute.ly, Scarlett - ITA!
I do think she wanted to disable him pretty quickly so he wouldn't have much of a chance to come at her or to defend himself. The chest wounds would have really slowed him down right from the start, and his blood loss would have been very disabling very quickly. Maybe that is when he somehow got to the sink, spraying his lifeblood everywhere and trying to get his breath, thereby exposing his back to her.
Then came those quick bad wounds to his back; now he is down & crawling away from her. Then the most-horrible-wound-I-have-ever-seen (and this includes the M.E. photos of the victims of the Manson family) to his throat. And I can just hear her now, saying "Take that, you ^(*&%^. That'll show you to break up with me! I am going to kill you, you %)*&^*)$."
If she attacked him from the back first, it would have surely stunned him right from the start, and she could do those without him seeing they were coming; start him staggering from the get-go. And then the rest.
And yes, Mz. Scarpetta, the gunshot was the period at the end of his sentence -- the sentence she pronounced on him due to her having to have the last word, scorned woman that she was. She had to have been standing over him, facing the right side of his head. Sometimes being left-handed does have its advantages, I guess.
I don't think he refrained from defending himself because he didn't have the heart to defend himself. Certainly, he was a gentle, nice guy; way, way above her on every measurement there is, but he would have fought for his life, but she was too mean, too quick, too jealous, & and too full of hate & evil.
He never had a chance.