There has been debates in recent years about the extent of the massacres conducted by Zhang Xianzhong. Traditional historiography, particularly that produced during the subsequent Qing dynasty, paints him as a brutal murderer who massacred even women and little babies. Zhang Xianzhong's biography in the Ming Shi estimated that he killed a total of "around sixty thousand thousand men and women" in Sichuan [將卒以殺人多少敘功次，共殺男女六萬萬有奇], or 600 million total. The Ming Shi wrote that he sent his four generals to every prefecture and county to massacre the population. He would unleash hounds into a crowd and whomever the hound smelled, that person would be beheaded. He also flayed captives (would make the Boltons proud), and if the captive died during the process, the person flaying him would be killed. It also recorded that several of Zhang Xianzhong's subordinates were unwilling to massacre civilians and so hanged themselves. "Zhang Junyong, Wang Ming, and tens of others, because they did not kill as many people, were flayed, and their entire families killed."

So the official history of the Ming painted Zhang Xianzhong as someone who was very brutal. But what is recorded here is actually quite tame compared to the things recorded in private histories. The Shu Bi, written in the 18th century, gave some very lurid stories about the massacres committed. Here some examples:

贼每屠一方，标记所杀人数，贮竹园中。人头几大堆，人手足几大堆，人耳鼻几大堆。所过处皆有记。 Every time the bandits committed a massacre [somewhere], they would mark the number killed and store [the bodies] in the bamboo gardens. A great pile would be made for the heads, a great pile would be made for the hands and feet, a great pile would be made for the ears and nose. Everywhere they went, they would leave such markers.

贼遇病弱者，多割鼻斫手。斫手之令，男左女右；若误伸者，两手俱斫。至小儿幼女，弃道旁衬马足；或掷之空中，以刃迎之。 Whenever the bandits met the old and infirmed, they would slice off the nose and chop off the hands. Whenever they chopped off the hands, the males would have his left hand [chopped off] while females would have her right hand [chopped off]. If they presented the wrong hand, both of their hands would be chopped off. As for little boys and girls, [the bandits] would abandon them by the side of the road and allow their horses to trample them; or they would toss them in the air and welcome them with their swords.

Most people would agree that this account is highly exaggerated, particularly since this was not an eyewitness account and was written almost a century later. But during the entirety of the Qing, this narrative of Zhang Xianzhong was supported by the government. By the early Republican period, however, views began to soften. The famous author Lu Xun, while not denying that massacres took place, did write that, "In the beginning, he [Zhang Xianzhong] did not kill that many people, he too wanted to become emperor. Later, when he heard that Li Zicheng had entered Beijing and later that the Manchus had entered China Proper, he realized that he had nowhere else to go, and so he started killing....He realized that there was nothing Under Heaven left for him, so he started destroying things that belonged to other people, this is not unlike the state of mine of some of the elegant last emperors, who, before their deaths, burned the collections of books, antiques, and treasures that they and their ancestors had collected. He [Zhang Xianzhong] had no antiques, but he had soldiers, so he killed, killed, killed, killed....Li Zicheng had already entered Beijing and became emperor, emperors needed civilians, so he [Zhang Xianzhong] killed civilians to make sure Li Zicheng could not become emperor."

We see here that Lu Xun compared Zhang Xianzhong to the last emperors of various dynasties. These last emperors were often heavily romanticized in later periods, such the last Ming emperor Chongzhen. In a way, Zhang Xianzhong is also being romanticized.

Modern historians, particularly in the Mainland, tend to portray Zhang Xianzhong as a proto-revolutionary. Hu Zhaoxi of Sichuan University wrote in his Distinguishing Zhang Xianzhong's Massacre in Sichuan that Zhang killed mostly Ming nobles, landlords, and counterrevolutionaries, along with their entire families. This is not unusual for peasant rebellions where landlords and men of power are often targets. He pointed out that Zhang was in Sichuan for only three years while Wu Sangui and Qing forces battled Ming loyalists there for almost two decades. Many today do believe that massacres took place, but not on the scale described in Qing sources, which heavily exaggerated the number of people killed and created nonexistent massacres. Furthermore, census data, which is often used to calculate casualties in a given region or the country as a whole, is highly unreliable because people could be displaced or not counted due to a variety of other reasons.

So given all that, it's more likely that Qing forces, in their battle with Ming loyalists, along with famine and disease, contributed to the depopulation of Sichuan as well, and that after the Qing victory, the entire blame was shifted to Zhang Xianzhong to make him a scapegoat.