It is true that Wei Yan is very capable, and Liu Bei's establishment of Shu-Han regime can not be separated from Wei Yan's help. But no one is perfect. Wei Yan is brave and has a good fighting style, but his personality is not smooth enough and he is arrogant. He offended many people, and others dared not oppose him. Yang Yi ignored those. Yang Yi hates Wei Yan and has always been at odds with him.
Wei Yan was appointed as a striker during Zhuge Liang's Northern Expedition. Soon after, Zhuge Liang was seriously ill and died. Yang Yi temporarily took Zhuge Liang's place. In fact, Zhuge Liang said before his death that Wei Yan would rebel after his death, which shows that Zhuge Liang has seen through Wei Yan's character.
After Zhuge Liang's death, Yang Yi followed his plan to find out whether Wei Yan had any thoughts of rebellion. Later, because the army retreated under Yang Yi's command, Wei Yan was very dissatisfied and refused to listen to military orders. He took the soldiers to Yang Yi's army and broke his retreat.
It is fatal to mobilize troops without authorization in violation of military orders. It also aroused Yang Yi's dissatisfaction with Wei Yan. At this time, the other party was falsely accused of rebellion, which finally caused confrontation between the two sides. However, due to Wei Yanxian's fault, the soldiers behind him did not listen to Wei Yan's words to Yang Yi. In the end, Wei Yan could only escape with a few relatives.
On the way to escape, Wei Yan was killed by Yang Yi's subordinate Ma Dai, and Yang Yi ordered the extermination of his three clans. In the final analysis, this matter is caused by Wei Yan's character. Although Wei Yan is a valiant soldier, he has a bad temper and is impulsive. When he chose to confront Yang Yiming, it was doomed that his ending would not be good.