Depending on who you read he is to some a hero. In the original Chronicles of the Three Kingdoms he was a man who wanted to unite China. However the adaptation written hundreds of years later, the Romance of the Three Kingdom portrays him as a villains we all know...
Actually I do not think anywhere was Cao Cao protraited as brutal. He is protraited as a villian mainly because he practically hold the Han emperor as his puppet and to control the rest of China.
Actually at that period of time, loyalty to your ruler is taken as the highest priority... even if your ruler is weak or practically useless. Therefore to hold the emperor as a puppet is regarded as a very evil thing to do, therefore Cao Cao is called a villian.
However if we look at things now and with today's context, if a ruler... or president... is basically useless and weak, he or she should be sacked and another should take his or her place to lead the country, no point supporting someone that is going to get the nation destroyed. So if you actually take today's context, Cao Cao is hardly called a villian. In actual fact you can call him an opposition party leader.
