Why did Sun Quan kill Guan Yu?

Sun Quan killed Guan Yu, on the one hand, to please Cao Cao's forces, and on the other hand, to destroy Shu.

According to "TongZhiJian", when Guan Yu besieged Fancheng, Sun Quan wrote to Cao Cao, requesting to work for the imperial court, allowing him to crusade against Guan Yu, and demanding that the news be kept secret. Of course, after weighing, Cao Cao hoped that Sun Quan and Guan Yu would kill each other and untie the siege of Fancheng by the way, so he passed the news to Guan Yu, but Guan Yu hesitated and refused to evacuate, and finally defeated Maicheng.

After Sun Quan killed Guan Yu, he immediately gave his head to Cao Cao.

Soon, Cao Cao appointed Sun Quan as the general of a title of generals in ancient times, saved Jingzhou as a shepherd, and sealed Nanchang Hou. In the past, Sun Quan only actually occupied the south, but now Sun Quan has nominal legitimacy.

Guan Yu launched the battle of Xiangfan, which embarrassed Cao Cao. Even if Cao Cao likes Guan Yu again, he must have a knot in his heart at this time. Besides, it is hard for Cao Cao's men to like Guan Yu. Therefore, Sun Quan's killing of Guan Yu has elements to please Cao Cao's forces.

More importantly, Guan Yu's position is extraordinary. Killing Guan Yu indicates that Wu declared war on Shu, and Sun Quan, Cao Cao and Liu Bei all know this. So Sun Quan killed Guan Yu and gave his head to Cao Cao, which definitely showed Cao Cao his attitude towards Shu Han, but on the other hand, it was also related to Sun Quan's strategic ambition.

The reason why Sun Quan killed Guan Yu may be related to Guan Yu's outspoken attitude, but the main reasons must be in these two aspects: on the one hand, it was to please Cao Cao's forces, to express Wu Shu's separation and confuse Cao Cao, so that Cao Cao could not intervene when Wu Jun quickly captured Yizhou; On the one hand, the ultimate goal of Sun Quan's plan is to destroy Shu, and killing Guan Yu is a normal operation.