Cao Cao has never been a man who can protect himself by any means. In the battle of Guandu, Cao Cao adopted Xu You's strategy and let Cao Cao sneak attack the Wu Dynasty. Cao Cao was overjoyed and rushed out to meet him. After that, he led an elite army of 5,000 people, who took the gold medal and put on Yuan Jun's uniform, and attacked Wu Chao from the alley at night.
So your personal starting point is wrong. Cao Cao has always been a man who wants to unify the world. Besides, Cao Cao was still young when he assassinated Dong Zhuo at that time, and it was not surprising that he did it with his character.