Soviet Union - The Red Terror, The Great Purge
China - Land Reform, The Great Leap, The Cultural Revolution
Cambodia - Khmer terror
Bulgaria
East Germany
Romania
North Korea
Vietnam
Ethiopia
Hungary
Total death toll between 58-94 million
I think the mass killings under Communism far outweigh any claims to being a bulwark against colonialism/imperialism. It was a nice slogan till the killings began.
I elide other nations, because I don't know the very details.
China
Land Reform: in the condition, land reform was very necessary. Nationalist Party's fail was mainly due to the wrong land policy. When Nationalist Party moved to Taiwan, they started to change and accept the similar land reform. If they did that in mainland, they wouldn't lose government.
The Great Leap: this is a totally wrong policy, I admit. China at that time didn't have the technology level, they had the strong will, but didn't have the ability. Fortunately, when government realized the policy was wrong, they stopped it immediately after 2-year operation.
The Cultural Revolution: This is really two-sides thing. The revolution improved many bad Chinese traditions: such as gender inequality,such as some strange superstitions; the negative side is breaking down elite cultural, low tolerance to dissenting opinions.
China didn't have what you state mass killings. In the beginning of 1960's, China broke out famine crisis due to climate disaster, USSR stopped assistance, focus on industry, etc. By the way, you please introduce some information about the great famine in India before 1970's which caused 10 millions Indian die.