I don't understand why this guy massacred people in his own province. Especially when the Manchus were running around conquering everything. And Sichuan was the only province he had. Why the hell did this guy suddenly decide to go apeshit on his own people?
Again, figuring out what happened in the whole affair is difficult. The population of Sichuan was quite low when the Qing got around to checking it after the war, and Zhang Xianzhong almost certainly did use terror tactics to stay in control of the province. But the more colorful tales are likely a combination of exaggeration and outright fabrications meant to make Zhang a scapegoat. The most likely culprits in the dramatic population drop are the hardships of war, and the Qing themselves, who had no problem getting nasty when they thought they had to.