Your question is somehow very unspecific.
Generally there are three forms of Amount of Rulers: One, Some, All citizens and the differentiating them by Altruistic(good) and Egoistic(bad) introduced by Aristotle.
This creates a table of the Form:
Altruistic | Egoistic
One: (Philosopher)King | Tyranis
Some: Aristocracy |Oligarchy
All: Polity/Democracy | Ochlocracy/Democracy
Therefore the Tyranis(Dictatorship) is generally separated from Democracy based on the Amount of citizens having a say and possibly the Form (depends on your understanding of Democracy). The Dictator focuses on his Egoistic Goals enriching himself on the cost of the citizens.
The understanding of Democracy varies depending on who you read. While Aristotle sees Polity as the legitimate form and Democracy(random selection of officials common in ancient Athens see: Demarchy) as the illegitimate form. Polity is understood in this context as Constitutional Parliamentary Government decided by enlightened citizens. This section is mainly to show the broadness of the Terminology.
I support more the distinguishing with the Terms of Polybios to address the modern situation, where Democracy is a positive Term. In opposition to democracy you have Ochlocracy or "mob-rule" as the negative form of "All".
We arrive at grave Problems when looking at communism, since A) communism is again a very broad often used and misused term and B) establishing the Form of Communism.
I want to focus first on B) here. Polybios describes an Anacyclosis(cyclical theory of constitutions/political evolution). Since communism sees itself as "freeing of the cycles of the past" it is debatable if the forms used to describe historical cyclical occurring patterns can be used for communism at all. Or in other words if forms of state organisation can be used to describe something that's not a state.
(Now comes the rather personal understanding of me part.)
However if we view forms as relational structures between a group of people the application is justified. To simplify matters in this regard it seems to be the easiest to conceptualize communism as special form of anarchism.
The relations between the people are power relations. Power has the characteristic of not being distributed equally in any given time. Furthermore power can be subdivided into authority(accepting someone freely) and violence(usage of physical intimidation to generate acceptance). State forms organize the power distribution in a certain way using usually authority and violence but to different degrees. Anarchism/Anarchistic states(f.e. revolution or civil wars) (often) do not organize or organize to a lesser degree. Leading to a more "law of the jungle/mob-like" power structure that is prone to demagoges (Robespierre/Hitler/Lenin/Mao/Pol-Pot/Kim Il-sung/Mussolini ect.). Making it's form the bad "All" form, where basically anyone can rise to power, where traits needed are bruttality and deception to hold your position.
Polybios describes in his Anacyclosis that 1 monarchy becomes(->) 2 Kingship -> 3 tyranny -> 4 aristocracy -> 5 oligarchy -> 6 democracy -> 7 ochlocracy ->1. I however think the 7 -> 1 part is questionable. It seems to be the case that out of 7, 3 arises aswell/or at least rather quick. Since you need violence to cement your position. (See Party Purges under lenin 1921, aswell as founding f.e. the cheka)
We can therefore conclude that communism understood as anarchistic is a very unstable state of powerrelations that tends to lead to tyrany and violence seemingly following the Anacyclosis more or less.
However to have a bit a clearer understanding of communism we should look at it a bit more closley (A).
Communism is an Idealism, based on an idealized view of reality(humans are per definition good, power can be evenly distributed, ect.) and fundamentally fails at this part. Positive intention does not implie positive outcome. The supporters of this totalitarian ideology fail to see the necessary leading to totalitarianism due to the idealized nature of communism. Because the supporters imply that the positive intention is sufficient. The idealization itself makes communism force people to being something their not, demanding a control and forcing structure (communistic parties) that guarantees that they stay something they are not. This structure accumulates power and as history shows missuses this power(f.e. killing mentally retarded people for "not being equal enough").
Consider that communism is not based on the usage of violence(violence is only used to transition to communism), since violence seems to imply an illegitimate state like powerstructure. It rather is based on authority of certain ideas that should be morally ensured. However we know that based on variations of Individuals in morality of Populations that there exist extreme cases of moral frameworks despite the moral dominant frameworks not supporting it at all (f.e. serial killers, pedophiles ect.).
If the state doesn't exist as a framework of violent power negating/punishing this acts of violence by individuals, theres either no structure to prevent murder, or it is based on initiative of self proclaimed protectors, that themself have a specific moral framework which themself justifiy their violence and accumlate power. However unlike democracy that limits the competence of the people with justification to use violence, this is not the case for the "communist case" where it's unrestricted and mob-like vengeance actions like killing the pedophile/serial-killer can take place. Which itself leads to overextensiv use of force generally leading to a tyranny of "one" or "some" powerfull people. The acummulation of power does never end since the idealized state is never realised, therefore one can always justify new restrictions torwards possible opponents by drawing the lines closer and closer to ones personal view.
So a communist state is an utopia that can not be reached. It also seems to attract people who see themself as godlike figures that could, if only given the chance, create a real communist utopia and cure the world. Overestimating themself by idealizing themself, this is not only arrogant but also false. There are plenty of countries that have become communist states, they all are expieriencing simular outcome despite vastly different cultural backgrounds. If the result is always the same thats what communism looks like. Just because it's not how you expected the process to turn out doesn't negate the fact that it is what happens when you implement communism.
The result is that it is a failure with tens of Millions of Dead people in just one century, everytime leading to totalitarian dictatorship and godlike person cult. Breading on the positive intentions of the naivie dreamers to create a fall of the current structure to allow powerhungry maniacs, that don't want to obey conventional moral/cultural laws, to rise to the top, and rather use violence to achieve their goal namley being in power.
Making Democracy a just and legitimate system, while communism is a demagogic illegitimate system that leads to unnecessary harm and death.
I personally hope this ideology is dead for good.