When Monroe crossed the river to attack Jingzhou, she defected and surrendered to Soochow, which directly led to the fall of Guan Yu's defense zone and his lair, and then defeated Maicheng and was killed.

Personally, I think that his surrender was only a temporary craze, and the bewitch of Wu Dong and Fu Shiren also played a great role. There is a simple reason. He doesn't need to do this. He is more important in Shu.

Later, he really regretted it and was frustrated in Wu. His eldest brother, Mi Zhu, was very unhappy and worried to death when he heard about him. He himself regretted it in Wu, and was not trusted by Dongwu. He was always worried that Soochow would send him back to Shu or kill him. He struggled in this regret and fear for several years in Wu, and died of depression.