So if she has no memory of where the knife was, she could have even gone to get it from downstairs.
No of course she didn't want to leave him like that - she would have covered up much better.
She was forced to attack an unarmed naked person, with a gun he supposedly owned, but did not reach for - she chose to get a deadly weapon, and to use it. He did not.
She could have yelled 'stop, or I'll shoot'. Backed out of the house ...anything.
She chose to kill him. Her choice. She chose to cover it up, consciously.