Because of his controlling mother, the former dowager empress (empress regnant) who used him as a puppet emperor. There may or may not be poison involved that caused his already unstable mind to deteriote even further (I can't recall). The painting on the was of a young maiden and her maids, and it people were of split opinion with one side believing the maiden to be his wife (the current dowager empress) and the other believing her to be a young version of his mother (the empress regnant).
I think the poison was in the paint he used. A lot of paint in the past was very toxic, and as he would close himself in the room with his supplies - he basically inhaled all those toxins.
It is also believed (I think) that he already had some form of mental illness - though that could be due to his upbringing with a controlling mother.
Can someone explain the situation about the late emperor's dislike towards older women? Or what he painted on the wall?