Zhang Xianzhong killed 75% of the people in the Sichuan province of China during the 17th century. There were around 3 million people in the area, and he killed 3 out of 4 of them. He did not use gas chambers, machine guns, or any sort of industrial tech since it hadn't been invented yet.
"The events surrounding Zhang Xianzhong's rule and afterwards devastated Sichuan, where he was said to have "engaged in one of the most hair-raising genocides in imperial history".[19] Lurid stories of his killings and flayings were given in various accounts. According to Shu Bi (蜀碧), an 18th-century account of the massacre, after every slaughter, the heads were collected and placed in several big piles, while the hands were placed in other big piles, and the ears and noses in more piles, so that Zhang Xianzhong can keep count of his killings.[20] In one incident, he was said to have organized an imperial examination ostensibly to recruit scholars for his administration, only to have all the candidates which numbered many thousands killed.[21] In another, to give thank for his recovery after an illness, he was said to have cut off the feet of many women. The severed feet were heaped in two piles with those of his favorite concubine, whose feet were unusually small, placed on top, and these two piles of feet were then doused in oil and set alight to be what he called "heavenly candles".[18]
He was also reported to have ordered further massacres before he abandoned Chengdu in advance of the invading Manchus.[17] The massacres, a subsequent famine and epidemic, as well as people fleeing from the turmoil and the invasion of the Manchus, resulted in the depopulation of Sichuan."
http://en.wikipedia.org/wiki/Zhang_Xianzhong