There were things that even the greatest powers couldn't change. There was nothing – nothing – she could have done to prevent it. The world had its own balance, and to force nature's hand out of their chosen path always meant a great risk.
To create a life, another should be taken, and Nimueh couldn't control whose life would be claimed. Uther couldn't see that – he had always worshiped her, and seemed to forget that she was only a priestess, instead he saw her as if she was the Triple Goddess herself, as if it had been her choice to take Ygraine away from him.
What could she possibly achieve by killing Ygraine? No, if she wanted power, she would have directed it to Uther, who was too blind to see that the powers of nature weren't hers to command, she could only bend them a little.
If she had the power, she would have taken any other life – someone that wouldn't be missed; someone too sick to recover, someone for whom death would be a blessing. If she had a say in which life would be claimed, she would have offered her own if it meant her best friend would be safe and happy, with her much desired baby in her arms.
Nimueh alone knew the price she paid to use such power. In the dark hours of the night, the cup of life would sing to her the songs of the dead, and when she sun shone on it, she would hear the laughter of children that would never be born. It was her burden and her blessing, as Arthur would be to Uther, and she learned to bear it, even when it left a bitter taste in her mouth, and so must he.
(He didn't.)