This article inquires into the puzzles concerning the emergence of evil() in ‘the study of human nature and principle’(). The Confucian philosophy of mind and human nature is generally based on the supposition that human nature() is intrinsically good(). However, in fact, human may have evil thoughts and act evil conducts. This fact forced Yulgok(谷) to insist that more evil affect or emotion() than good affect() are emanated out of human mind. On a par with his insight, this article explicates the question whether there is in human nature the cause of evil, and the question how it is possible that more evil affect than good affect are emanated out of the innately good mind. In particular, we firstly criticise the conceptions of evil affect Yulgok constructed, and then try to analyze the puzzle of evil affect from the view point of ethical antecedentialism and consequentialism. Secondly it will be explicated that the conceptions of affect(), intent() and passion() have relationship with the emergence of evil, and finally ethical traits and significance of intent is explained from respects of moral will.