Communism is commonly associated with dictatorship because dictators almost always choose communism as their form of government when they come into power.
Communism rejects democracy. So it rejects the existence of multiple political parties and the ability to vote for them, but it also rejects traditional democratic control mechanisms, such as like separation of powers, that were specifically designed to prevent democracy from degenerating into an autocracy.
If leaders kept communism about sharing knowledge and wealth equally among the people so everyone could get a fair shot, communism would be great. But, all that powers quickly goes to heads and it becomes a popularity contest where the liked are favored most.
