Unlike many dynasty-ending (or nearly dynasty-ending) rebellions in imperial China, the An Lushan rebellion interrupted a period of relative prosperity, rather than occurring at the end of an era of steep decline (e.g. the Red Eyebrows at the end of Xin, the Yellow Turbans toward the end of the Eastern Han, or Huang Chao's rebels near the end of Tang). Under Emperor Xuanzong, Tang had enjoyed geopolitical hegemony in East Asia and on the steppe, a cultural and literary golden age, and vibrant international trade sustaining its bustling and cosmopolitan cities.

If you were a Tang official of long and distinguished service, like Zhang Xun, you were likely deeply loyal to the Emperor, his dynasty, and the institutions of the state that sustained all this splendor. In his analysis of the mid-Tang (post-An Lushan) poet Li He, Changming Yuan discusses the patriotism of Chinese officials of the dynasty, differentiating it from the patriotic nationalism of the modern era while making a convincing argument that Tang officials felt a strong sense of duty and loyalty to their country. This patriotism, later in the Tang during times of trouble, was expressed through efforts to secure the dynasty's preeminence over other state actors (in Li He's time, restive regional governors).

It is highly plausible that an official like Zhang Xun, living at the close of a more prosperous era, was motivated by an intense form of patriotism. His career in the Tang civil and military service (the latter entailing campaigns on the frontier) had been distinguished, if reflective of some eccentricity and single-mindedness. Indeed, his actions before Suiyang - to include leading fellow officials first to worship the ostensible Tang ancestor Laozi in defiance of a Yan-aligned superior, then in revolt against the usurpers - suggest that he was incorrigible in his defiance of Yan. Yan, as it happened, was the creation of the general An Lushan, someone whom an official like Zhang Xun might have found entirely unpalatable as a ruler. An Lushan was one of several foreign-born frontier generals favored by the former chief minister Li Linfu, likely because empowering them served as a counterweight and alternative to elevating aristocratic literati at court who disagreed with Li Linfu's autocratic tendencies. Illiterate, uncouth, and known for his brutality, An Lushan nevertheless possessed a certain charm and found his way into the good graces of Emperor Xuanzong and his consort Yang Yuhuan, who even "adopted" An as her son in a farcical ceremony.

Such an individual, existing far outside the mainstream of officialdom, was hardly likely to garner good will from the official literati (Zhang Xun, for instance, was respected for his literary skills), and far less after he betrayed his perceived benefactors at court and openly rebelled. The notion of An Lushan (or his successors, who were ruling Yan at the time of Suiyang) supplanting the golden age of Tang would doubtless have proven a bitter pill to swallow for established imperial officials, a class to which Zhang Xun belonged.

Whether Yan was truly some evil, despotic state is another question altogether. It was certainly not Tang, and as the recent, rather hodge-podge creation of military men raised suddenly to the ranks of royalty, it had a lot of rough edges. And in the case of Zhang Xun, the historical evidence suggests that he was proud (to a fault) and fanatically loyal to the empire that he had served for years. Like many in a similar position before and after, he was faced with the choice of surrendering or fighting to the death, and chose the latter. The patriotism that Zhang appears to have exhibited would not have been a reciprocal sort of loyalty - the Emperor was the ruler of all under heaven, and whether he sent troops to your aid or not, you owed him your allegiance if you were a good and faithful official. One is reminded of the late-Western Jin dynasty tale of a leading minister who was maneuvered into commanding a suicidal military operation by a rival at court, but carried out his orders nonetheless, asking, rhetorically, what else could a loyal minister do? And there was a long tradition, in Tang and in prior dynasties, of lauding officials who had loyally served flawed emperors, often at their own expense (even at the cost of their lives). So in this regard, by choosing not to surrender, Zhang Xun was not doing anything that would have seemed at odds with the traditionally perceived prerogatives of a loyal official. Obviously, though, he went quite a bit further than that.

As for why Zhang Xun went as off the wall as he did - cannibalism - it's likely a corollary to his fanatical sense of patriotism, and perhaps a recognition that he and his men were fated to die at that point anyway, so anything that could be done to defend the city and foil Yan's broader strategic ambitions was worth it. Notably, his approach had its detractors even among those who shared Zhang's affinity for the Tang cause, such as retrospective commentators, given the obvious moral and ethical implications.