If you try to prove that Socialism or Communism doesn't work, you are able to point to myriad examples of where these systems failed. The Soviet Union, China, Venezuela, and more stand out as shining examples of the economic malaise and social misery that ensues once the socialists take over.
The communist defenders of the faith immediately reply "that wasn't pure Socialism" or "that wasn't pure Communism". They claim that all of these failures were because the ideal of Communism was imperfectly executed, and that if only they had been more faithful to the utopian ideal, they would have had the glorious results that Marx promised.
The problem with the utopian reply is that we will never have perfectly implemented socialism. Why? Because political theories have to be implemented in the real world by real human beings. Flawed human beings that will never live up to the ideals of the theory.
Communism fails for a number of reasons, but arguing that it fails because of the imperfect people doing the implementation leads to mass murder.
All human beings are flawed. We are greedy, selfish, lazy, stupid, venial, and petty. We will never be able to achieve the ideals of any belief system or economic system. This applies to both socialism and capitalism. We live in the real world and must judge the outcomes of our economic theories on the actual results, not the utopian ideal.
The communists realize that this is true. They understand that men are flawed and that socialism won't work with the current human race. This is why the Soviets created the ideal of the New Soviet Man. The 'new man' was to replace the 'current, old man' in order to make socialism work.
In practice, the socialist and the communists are too impatient to wait for the New Man to arise. There are too many naysayers and malcontents that are 'sabotaging' the system and are 'counter revolutionaries'.
The solution is simple and obvious: Kill all the unbelievers.
They set about murdering all the people that are too 'flawed' to become the New Soviet Man. 30 million Kulaks killed in Russia. 1.5 million killed in Cambodia, including people killed for the crime of wearing glasses because they 'looked educated'.
Socialism and Communism can never work, but the ardent acolytes of this flawed ideology blame people for not living up to the standard required to make it work. So they kill them.
The next time you hear a communist tell you that pure communism will work, understand that they are planning to kill hundreds of millions of people to prove it.