It's simple.
A large civilization can only exist if its laws are enforced. Imagine a place like NewYork City, without police.
What are police officers? The bottom rung enforcers for the government; The arbiters of that use of force. The thing here that you are NOT grasping--- More people will be OPPOSED to communism's needed reforms, than would be in favor of it, because it is NOT the natural predilection for humans to hold the necessary views. Getting humans to override their base impulses is very hard; See for instance, getting diabetics to lay off the sweets.
Visionaries often times neglect to appreciate that biology is very much a part of this problem. you can't just ignore it, or expect to treat it satisfactorily with education alone. Imposing communism is about at the same level as imposing veganism. Yeah, there are people who are mentally dedicated to being vegan. They are not a majority.
The difference between "amiable rule of law" and "Tyranny", is the percentage of the population that agrees with the policies being enforced. Nothing else. Democracy helps to ensure the former, by enabling the majority of the population to define the parameters, rather than a dictator. Again though, the problem you will have, is that most people WILL NOT WANT the needed changes of communism; Their internal biological programming will be too strong to override with pure reason.