How does Miss Havisham explain why she did not correct Pip's belief that Estella was intended for him?

She says it wasn't her duty to be kind to Pip or lead him down the right path, "Who am I,' cried Miss Havisham, striking her stick upon the floor and flashing into wrath so suddenly that Estella glanced up at her in surprise, `who am I, for God's sake, that I should be kind?"