Admiral James Kirk wrote:
Actually Zinaida willed herself to die when Khan attempted to plant a Ceti Eel in her. Though later in the novel it said all the scientists had their throats cut. I suppose it's possible he cut a dead woman's throat just to horrify Kirk later on. That's doubtful though since he intended to kill Kirk before he reached Spacelab.
I assumed he mutilated the body a) to make sure and b) to take some kind of vengeance on a woman who defied his wishes.