Nolan was trying to protect David Clarke's legacy because they were filled with seemingly paranoid thoughts. They also contained an entry claiming that "CM" will come with the truth. Emily realized that the initials stand for Conrad Grayson's secretary, Carole Miller.
Emily does a little digging to learn that Carole Miller is actually very much alive. She's living under the name Carole Thomas in Lancaster, PA.
She was caught off guard when she sees a picture of the woman with Nolan. This is moments before Carole points a shotgun at her. Nolan came barreling down the stairs to tell his Aunt Carole to lower her weapon. Nolan is the nephew of Conrad Grayson's ex-secretary. Carole was the person who introduced him to David Clarke.
She started digging around Grayson Global and her search led her to a senior accountant named Roger Halsted. Roger had discovered enough evidence to hang Conrad Grayson and exonerate David Clarke, but David was murdered before anything could happen. Nolan faked his aunt's death to assure her safety.