In 20th century the communism was the biggest killer
This is quite a controversial statement. USA threw the nuclear bomb on the citizens twice... Also they used chemical weapon and another weapon of mass destruction many times, while USSR never did anything like this.
Fair enough. To clarify: communism was not only in USSR, you need to add China, Cuba, Cambodia, Yugoslavia, Vietnam and many more countris that had/have communist system. Death toll is enormous mainly because of mass killings and famine that followed after nationalizing everything - change of system.
Since this is thread about Ukraine here is an example: starvation in Ukraine that killed few millions in 2 or 3 years. Similar thing happened in China, except the numbers are ten times bigger.
http://en.wikipedia.org/wiki/Holodomorhttp://en.wikipedia.org/wiki/Great_Chinese_Famine