Mary told Keir to learn lesson which life teaches you and to learn that not to be so rigid about right and wrong.
Step-by-step explanation:
In the book named Inexcusable, when Keir dodged a bullet, Mary told Keir that he should have been careful with things and not take the lessons lightly which are taught to a human being by the life.
According to Mary, Keir should learn to be tolerant and not to be so rigid about what is right and what is wrong. Keir should learn that not every thing in life is either black or white. She should take the lessons learnt from life very seriously.