Disclaimer I do not own Harry Potter
At first it was lust of power that brought her to his bed, then it was desire for him, but in the end it was love that brought her there.
It amazed him, what she was willing to do all in the name of love, without remorse she'd torture and kill, All in the name of love.
In others he found love was a weakness, but it made her stronger,
it made her a better warrior.
It was her love, her loyalty, her desire to please him, that always saved her from him punishing her for failure.
Even Evil wants love.