I would like to just add to +1 answer of @fizz.
It is completely true that dictators often are self interested but the problem with communism is not only this but the fact that many ideas that are according to self proclaimed communists at the core of communism are are inherently inefficient.
For example, many communists want to get rid of private property (See the history of economic thought by Grant and Brue).
But a problem is that private property is an irreplaceable institution necessary for economy to prosper (see any economics textbook for example Mankiws Principles of Economics). In fact the famine in communist Russia is often attributed to farm collectivization which took the ownership of the farms from farmers and put them into hands of state where people had little incentive to take well care of them. This being said some level of economic output is attainable even without property rights, pre-historic hunter-gatherer societies existed without them, but it is generally agreed that they are necessary precondition for sustained economic development (See Acemoglu & Robinson Why Nations Fail). Also, lack of property rights is actually the most cited cause of our current environmental problems i.e. over-pollution, over-fishing etc (again see Mankiw's Principles of Economics) as absence of property rights creates the tragedy of commons.
There reason why lack of property rights leads to loss of output and efficiency is that without secure property rights people cant be sure they will be able to use their output in any way. A farmer would not produce any output if all crops are expropriated. Furthermore, due to tragedy of commons in absence of property rights people are locked in prisoner dilemma where everyone is incentivised to overuse resources before anyone else has chance to already collect them. There are also other issues and this is only short and surface level exploration of importance of property rights as full explanation would be way beyond the scope of this answer but I encourage you to explore it more fully in the sources I provided and works cited therein.
Moreover, communists often believe in radical egalitarianism (Again see Grant & Brue). As Marx himself wrote “from each according to his ability, to each according to his needs.” Now there is nothing wrong with believing inequality is bad and trying to reduce it but what many self proclaimed communist fail to grasp is that inequality is what gives people incentive to put in extra effort, and hence any society that wants to increase its standards of living has to have some inequality. If someone works hard and gets the same pay as someone who slacks of then what’s the point? People soon learn it’s better not to put extra effort - and consequently many communist countries eventually result to some sort of forced labor which is again much less efficient than free labor where people are properly incentivized. This does not mean that every single action in an economy needs economic incentives (narrowly defined) there can obviously be instances of altruism because peoples' utility can take into account the well-being of others (as famously alluded to already by Adam Smith in his Theory of Moral Sentiments). Moreover, people also can intrinsically value some work in itself - if you have any hobby where you produce something that would be an example of that. But it is generally agreed upon by profession that no large economic system can function without a system that provides an economic incentives for people to produce goods and services optimally.
Even the goal of the proletariat dictatorship, in the Marxist-Leninist tradition is supposed to be true communism that is property-less, moneyless (i.e. based on barter and communal exchanges), classless (i.e. there are no workers no owners no classes at all) and stateless (Marxism is in its hart anarchist movement as state is only seen as temporary apparatus that dies out once the true communism is achieved).
However, as already mentioned above property, inequality (which implies some sort of class) are important. Money is also important in economics as it provides a solution the issue of double coincidence of wants which otherwise wastes precious time and other resources, and economists agree state is necessary for provision of public goods and very helpful in solution to many problems that are based on externalities. So even if we could achieve the true communism it would probably fail to deliver high material welfare standards.
In fact the only time in human history where the sort of society that Marx thought communism would be was actually in existence, was in prehistoric hunter-gatherer societies which were always living at subsistence level.
Hence often unappreciated fact about communist regimes is that they would often not work even if their leaders would be angels. To give you analogy, no matter how a good person you are if you want to cure people using homeopathy you won’t help them.
However, you should also note that communism as mentioned by Giskards +1 comment is badly defined. In economics communism does not even have a definition - you won’t find the term in many modern economic textbooks. The term communism is so often used and misused that it is meaningless. Trying to define it is as trying to define a deity - nobody will probably ever agree on single definition. Here I use the term communism as a system that would describe the ideologies and economic goals of Soviet Union or Maoist China.