Personally, I think that she intended to kill him with the knife and only had the gun as back-up. I don't think she wanted to use the gun because of the fear that someone would hear it fire, and she might get caught in the house by police or a neighbor. I also believe that she imagined she could stab him once or twice and he'd die neatly in the shower with all the blood evidence being washed away. I think she freaked out after she stabbed him and he fought back, got out of the shower and was able to not only get out of the bathroom but all the way down the hall, and she went into a frenzy of stabbing because he wasn't conveniently dying right away. I also believe that the throat slitting and subsequent gun shot to the head was that she wanted to make sure he was REALLY dead, but also because she was pissed off at him for not dying neatly and quickly in the shower and made a bloody mess of the place which was not at all what she had invisioned.