Here's why:
* He created a female creature, but never completed her. He was horrified by the idea of a second monster and the potential for reproduction, leading him to destroy her partially formed body.
* The female creature was never alive. She existed as a partially assembled form, not a fully sentient being like the male monster.
Victor's actions in relation to the female creature were driven by fear and a desire to control the consequences of his actions. He never directly destroyed a living being in this instance.