For her entire life, she was powerless. Everything was decided for her from her meals to her education to her hobbies to her friends, and even her own marriage, not once had she ever been able to give input. The only power she was ever given was in order to end her own life when she was no longer useful. After the darkness overtook her vision, she swore that if she was ever given a new life, she'd be the only one calling the shots.
Finding herself in the body of a young, weak concubine-born princess confined to a tower, she smiles to herself for the first time in years.
Power is life, and without power you are nothing. Stepping on others, killing others, using and disposing of them, as long as it is in the pursuit of power, who cares?