Cao Cao's a traitor to...han and Dong Zhuo. But compare to Liu Bei he is nothing...not even close. Liu Bei was a traitor to Liu Zhang's father, Lu Bu, Cao Cao, Yuan Shao, Liu Zhang, Liu Biao, Sun Quan, and Han...if he's loyal to han, why don't he make one of Han's heir emperor instead of himself? (and the heir was not killed, just banished by Cao Pi).
So Cao Cao was a traitor, but he's normal in the era. The ones who weren't traitors were abnormal. Taishi Ci's a traitor. Gan Ning's a traitor. Sun Ce's a traitor. Lu Bu's a traitor. Zhang Liao's a bigger traitor. Xu Huang too.