Romance of the Three Kingdoms is a historical fiction based off the actual Three Kingdoms actual history (the Sanguozhi). Specifically, the RoTK is infamous for having an extremely strong bias in favor of Liu Bei and the Kingdom of Shu/Han, and an extremely strong bias against Cao Cao.

Examining the official history, we see that around the end of the period, the Han dynasty was in quite a bit of turmoil. The shit really hit the fan with the onset of the Yellow Turban rebellion. In order to combat the rebellion, the imperial government greatly expanded the military and political powers of local warlords in return for their help. These warlords defeated the rebellion, but in turn the imperial government control over these warlords had been dramatically weakened, as these warlords had all the military power. This led to a series of disputes over who could have control of the Emperor, as well as several waves of persons attempting to declare themselves emperor-including Liu Yu and Yuan Shu.

One of these warlords was Cao Cao. Cao Cao had a cadre of competent officers and troops, as well as a strong power base in the Central Plains, near the large population centers and near the Emperor. After subjugating the local warlords, he fought a major conflict with his rival Yuan Shao and defeated him decisively, thus ensuring his hegemony over the Central Plains, and by default, the Emperor. The Emperor and his Empress plotted to overthrow Cao Cao on several occasions, but these plots were detected and Cao Cao had the other plotters executed, for treason, installing his supporters and relatives in their place, further consolidating his hold on the imperial court. Unlike his son Cao i, however, Cao Cao had no ambitions on seizing the throne outright, as the Emperor had already made him the "king" of a vassal state, Wei.

Source: Sanguozhi