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Corrupting something so precious to the Doctor is fun. It's like winning every game.
She isn't like the Master. The Master liked to destroy anything the Doctor cared about. Childish and ridiculously stamped their feet to get The Doctor’s attention and affection. Destruction was the only way The Master knew how to do it.
She liked to make them alive and corrupted. The Rani knew the Doctor’s hearts. They'd break if they ever knew. Because he cares about this human. A deep almost familial type of love.
Yes, The Doctor was the reason she did this. Not because she enjoyed the way Kate gasped and cried out her name like a prayer. Not like the name of God, but something unholy.
It most definitely isn't because she enjoyed the tortured look on Kate's face at enjoying something The Rani had let Conard build a world where feeling like this over a woman was wrong.
She isn't fond of her. She's not. The Rani really isn't. Even if her previous regeneration insisted she was. What did she know just because she was her?
Just because when Kate began to break and have doubts, Rani gave her the one thing that would keep happy, her father alive. Hell she was going to destroy this world, why not make at least one person truly happy.