Here is a site with a lot of information on masses of killing. Below that is a list of worst killing events or periods. Given that Genghis Kahn lived in a century with a much smaller population than Mao’s century I rate him the worst.
http://necrometrics.com/pre1700a.htm
Rank Death Toll Cause Centuries
1 66 million Second World War 20C
2 40 million Mao Zedong (mostly famine) 20C
40 million Genghis Khan 13C
4 27 million British India (mostly famine) 19C
5 25 million Fall of the Ming Dynasty 17C
6 20 million Taiping Rebellion 19C
20 million Joseph Stalin 20C
8 18½ million Mideast Slave Trade 7C-19C
9 17 million Timur Lenk 14C-15C
10 16 million Atlantic Slave Trade 15C-19C
11 15 million First World War 20C
15 million Conquest of the Americas 15C-19C
13 13 million An Lushan Revolt 8C
14 10 million Xin Dynasty 1C
10 million Congo Free State 19C-20C
16 9 million Russian Civil War 20C
17 7½ million Thirty Years War 17C
7½ million Fall of the Yuan Dynasty 14C
19 7 million Fall of Rome 5C
7 million Chinese Civil Wars 20C
I’m not sure I understand why famine in India would be blamed on British colonialism.
What is unmentioned here is that about 100 million Indians perished from various plagues between 1850 and 1950.
The British trained and equipped thousands of Indian public health workers and Indian engineers in an effort to control those plagues, which had been present for centuries before the British arrived.