First of all, if you like the history of the Three Kingdoms period, I strongly recommend that you read The History of the Three Kingdoms. Although the records in the History of the Three Kingdoms may not be completely accurate, the History of the Three Kingdoms is the closest version to the real history at present, but many people like the Romance of the Three Kingdoms. After all, the reflection looks like a history book, and people may not like it very much.
Secondly, in real history, emperors in Tang and Song Dynasties actually worshipped Cao, and several emperors in Song Dynasty liked Cao Cao's poems and his way of dealing with people. However, a classic novel Romance of the Three Kingdoms appeared in the late Yuan Dynasty and early Ming Dynasty, which directly portrayed Cao Cao as a traitor. Basically, people who read the romance of the Three Kingdoms think that Cao Cao is a bad person.
Of course, Cao finally said that he can't be an absolute good man, but he is by no means a sinister person. Many descriptions of Cao Cao in Romance of the Three Kingdoms are really dark, which also affects everyone's judgment.