Cao Cao executed Yang Xiu because Yang Xiu always tend to show off his talent and sometimes even unveil Cao Cao's secret plan. There was one time when Cao Cao told his servants and officers that he will killed someone when he was dreaming. One day, when Cao Cao was sleeping, his blanket dropped to the ground. Cao Cao's guard who was afraid Cao Cao will caught cold, took up his blanket. Suddenly Cao Cao jumped up and killed the guard. Then, Cao Cao pretended he was still asleep until other guard called him up. Yang Xiu who knew this incident said openly to the dead guard,' It was not the Prime Minister (Cao Cao) who are dreaming, it was you who are dreaming,'. Here, Yang Xiu clearly unveiled Cao Cao's plan.
Anyway, the most important factor and reason why Cao Cao wanted to killed Yang Xiu was he involed in Cao Cao family matter. Cao Pi and Cao Zhi race to become the prince who will take the throne of Cao Cao. In this race, Yang Xiu helped Cao Zhi out. He gave Cao Zhi many ideas about how to win Cao Cao's heart and took the throne. First, Yang Xiu tried to unveiled Cao Pi's plan to hide Wu Zhi (吳質) into something like bucket before Wu Zhi can dicussed matter with Cao Pi at Cao Pi's palace. When Yang Xiu brought Cao Cao to check on those buckets, nothing were found (Cao Pi knew it earlier). So, Cao Cao thought that Yang Xiu wanted to accuse Cao Pi. Second, Cao Cao wanted to test Cao Pi and Cao Zhi how they solved problem instantly, they have been ordered by Cao Cao to go out of the palace after the forbidden time. Cao Pi tried first but was blocked by the guard so he retreated. Yang Xiu taught Cao Zhi to kill the guard because he had Cao Cao's order, Cao Zhi did so and Cao Cao delighted with Cao Zhi's work. Unfortunately, later Cao Cao found out it was Yang Xiu's idea, Cao Cao hated Yang Xiu even more. Third, Yang Xiu prepared some answers to Cao Zhi to face Cao Cao's question. Cao Pi who knew this used money to get those answers, he gave it to Cao Cao. Cao Cao knew it was Yang Xiu again who are flaming behind Cao Zhi.
When Cao Cao decided that Cao Pi should took his throne. He wanted to get rid all those people who against Cao Pi. Here, those who were helping Cao Zhi should be the first taregt to get started with. So, when Yang Xiu involve in chicken tendon's incident, he should be killed. This was one of the way for Cao Cao to prepare a road for Cao Pi to take over. I don't think he really wanted something happened awfully between all his sons after he passed away. Yang Xiu was talented and who knew maybe he would taught Cao Zhi to 'rob' the throne from Cao Pi. So, the best way to avoid this was get rid of Yang Xiu.
