tl;dr: If non-dictatorial communist societies existed, then not for long. But what is communist anyway?
There's a lot of confusion concerning the term communism. There have been many movements that called themselves communist, and more often than not they have denounced each other as wrong or traitors. Let's try to get a definition from the horse's mouth:
Communism is for us not a state of affairs which is to be established, an ideal to which reality [will] have to adjust itself. We call communism the real movement which abolishes the present state of things. The conditions of this movement result from the premises now in existence.
-- Karl Marx, The German Ideology (1845)
Marx puts the emphasis on the movement here, instead of dreaming up an abstract utopia. He denounces the idea of saying the world has to be such-and-such, and then implementing it. In fact, he went on to say "je ne suis pas Marxiste" - I am not a Marxist.
Still, we are interested in societies that have claimed to be communist, or Marxist, etc.. I like the following definition, with which Marx and many Commies hopefully would have agreed:
A communist society is one where the means of production are collectively owned.
I'll assume that at least on paper there is some kind of democracy, otherwise the whole phrase 'collectively owned' is silly. That means you and I can - in principle - decide what to produce though some kind of democratic progress, rather than leaving that to the market.
There have been several societies I would call communist in the past. Most were either in a religious context, or rather short lived. A few that come to mind:
- Early christian communites. People shared their wealth and put their possessions together, in expectation of the end times. It's a bit hard though to define what 'means of production' means in this context, as this was in pre-modern times. Also, I can't vouch for how democratic things were.
- Kibbutzim. In Zionism, prior to the founding of the state Israel, there was much discussion about what character the new jewish state should have. Secular vs. religious, socialist vs. capitalist, etc.. In the end, one settled on a capitalist state modeled after modern western countries. Many of those who had preferred a more communist direction went on and founded Kibbuzim to live their ideas.
- The early workers' soviets (= councils) in Russia. These were quickly deprived of their power by Stalin and the Bolchewists.
- The Paris Commune, which was tending between social democracy and more radical communism. I can't say in which direction it would have developed hadn't it been beaten down after a few weeks.
- Anarchism during the spanish civil war, particularly around Barcelona. Sadly and ironically, the actual communists (Stalin) did not support the elected Spanish republic against the fascists (Franco and Hitler), but fought against them as well.
Ok, but what about the big communist or socialist states (USSR, China, ...)?
Going back to the quotable guy with the beard. He begins the Kapital with:
The wealth of those societies in which the capitalist mode of production prevails, presents itself as "an immense accumulation of commodities," its unit being a single commodity.
-- Karl Marx, Das Kapital (1867)
So, it's the mark of a capitalist society that things are produced not just in any random fashion, but as commodities. That means they are made primarily to be sold on some kind of market. The laborers who produce them get paid a wage, with which they can buy those commodities.
The funny thing is that - according to that definition - most large scale communist societies were actually badly run capitalist societies! Badly run in the sense that instead of competition, you had one big monopoly, which was identical with the state. It has been pointed out many times that this is a bad idea:
But when economic power is centralized as an instrument of political power it creates a degree of dependence scarcely distinguishable from slavery. It has been well said that, in a country where the sole employer is the state, opposition means death by slow starvation.
-- Friedrich Hayek, The Road to Serfdom (1944, freely quoting Leo Trotzki)
So, summarizing I'd say two your two bullet points:
- Communism in the vein of the Soviet Union is certainly a failed idea. Communism in general however is too broad a term to prove 'failed' or 'neccessarily leading to dictatorship'.
- The idea that communism is an ideal that 'just has to be applied correctly' would have Marx rotating in his grave. If you just have an abstact idea, instead of a solid analysis of the society you criticize, you might just end up making the same mistakes. In fact, I believe this is one reason why the USSR turned out to be as terrible as it was.