She did it to free William. She was never sure about him to begin with. Now that he was sexing Tania, yukkkkk, why would Nikki ever let him into her bed again. She would never be able to trust him again anyway. She figures the way to get rid of him for good and save herself anymore pain is to just let him get to stepping. After all knowing had been with Tania, I wouldnt want him near me either.