The ideal would be a global revolution. When the Russians revolted against the Czar, there were worker revolts all over Europe, which were largely disorganized and failed. In some cases, (Italy, Germany, Hungary) the worker revolutions nearly succeeded but the capitalists started funding fascist elements, like Mussolini and Hitler to crush the communists.

Meanwhile, the newly formed USSR found itself facing armies of US, UK, France and Japan, as well as the White Army, which mainly operated in Poland and Ukraine, who were all aiming to crush communism before it spread to their countries. The radical measures taken during that period (mass deportations, drumhead trials) as well as the catastrophes (famine, disease) were spurred by a desperate attempt for the USSR to survive.

Same in Cuba and China during the first couple decades of the communists taking over. Most people who attribute all the oppression to the "inherent faults" of communism fail to take into account that communist nations throughout history found themselves isolated and facing multiple threats from all directions, which forced them to implement inhumane measures in order to survive. A similar situation arose in France after their republican revolution, but the executions during that era are not attributed to the "faults of democracy".

So as long as there are capitalists, communism can not achieve its potential. But the aim is not to kill all capitalists. The aim is to take over the power they hold, both industrial/economical and political, which will force them to get assimilated into the new society. This is not such a good analogy, but think of it in terms of slavery: As long as there are people who benefit from slavery, they will fight tooth and nail to keep slavery around. Likewise, a communist has no sympathy for a capitalist who struggles to maintain their position.