Apart from what I stated above about sadism and rage, maybe she had the gun just in case the stabbing was not effective enough and he came after her. Then she could shoot him if she thought her life was in danger.
But I am of the opinion she planned to have both weapons- in her mind she would stab him repeatedly and then effect the coup de grace, execution style. She wanted him to suffer and then to be crushed like an insect. She was a goddess all right. I think that's the way her mind and fantasies work.
I have a theory as to why 2 weapons were used: She thought that maybe it would throw off the investigators by making it look like there were 2 killers.
It's even possible that in her pre-meditation, she considered using the Ninja story, if necessary and 2 weapons would seem support that story.