Why did Zhang kill Sichuan?

According to the historical records of the Qing Dynasty, when Zhang retreated from Chengdu in the third year of Shunzhi in the Qing Dynasty (1646), in desperation, he carried out unprecedented burning and destruction in Sichuan. There are only 20 households with 400,000 people in Chengdu; Sichuan, the land of abundance, was devastated, and its population dropped from at least 3 10/00000 degrees to only 80000 degrees, which led to a large-scale immigration tide that lasted for more than a century in the Qing Dynasty. The so-called "Huguang fills Sichuan". In the same year1October 20th, 10, Zhang was shot and killed by the Qing general Hogg in Fenghuang Mountain, Yanting County. When the officials of Daxi left Chengdu, there were still 700 people. By the time Zhang died, there were only 25 people, most of whom were killed by him. The four-volume Book and Pen written by Peng Zunsi in Qing Dynasty described Zhang's brutal massacre in Sichuan in detail. Li Leisi and An Siwen, western missionaries who preached in Sichuan at that time, also had similar records in their book "The Story of the Holy Religion Entering Sichuan". Shen Xunwei said in the preface to southern Sichuan that Zhang was destroyed in Chengdu because of the pursuit and suppression of the Qing army. "Several palaces in Wang Fu cannot be burned down, but they are full of grease. Panlong Shizhu No.2, when Shu Meng was alive, wrapped dozens of layers of yarn, soaked in oil for three days, and broke the column with a fire. " The troops fled to Xichong, no one killed them, but committed suicide, with 10,000 to 20,000 people a day. But historians are skeptical. Zhang's massacre in Sichuan is true, but the scale of the massacre has not reached such a level. Many massacres in Sichuan should be done by Qing soldiers. Many years after Zhang's death, the Qing army was able to pacify the whole Shu state. 1647 Zhang died in Qing dynasty. 1649 A proclamation of Manchu people in Sichuan: "There are many thieves, but jade is hard to separate. Or slaughter the whole city, or slaughter men and keep women. " 1685, in the 24th year of Kangxi, the whole territory of Shu was pacified. "