Having failed to persuade Chiang Kai-shek to abandon war with the communists and instead unite to fight Japan, Zhang Xueliang and Yang Hucheng finally opted for force and captured Chiang in the Xi'An Incident, not releasing him until he had agreed to fight Japan alongside the CCP. In the aftermath, both were arrested by Chiang. Over a decade later, when the KMT retreated to Taiwan after the Chinese civil war, Zhang was taken along as well and continued in captivity for several more decades, while Yang was executed.
What reasons have been given by historians to explain why they received different punishments? Did the KMT give any explanation of why they imprisoned one but executed the other?