To be fair, I think that Gary Childress is asking a question of somewhat more nuance than Celtic Empire's accusatory demands, so I'll address it anyway.
Well, firstly, I'd advise being more careful about how the word "socialist" is interpreted. The Nazis used the term, yes, but they were also in practice and largely in word anti-socialist; the term had populist value, and their use was in the tradition of similar (albeit more passing) appropriations by Louis Napoleon and Bismarck. The National Socialist German Workers Party, it can be fairly conclusively said, was a fascist* party, not a socialist one, and the two are rather by definition mutually exclusive.
[*"Small-f fascist", which is to say, an ultra-nationalist mass-movement of the petty bourgeois with anti-communist and palingenetic fixations, as opposed to "capital-F Fascism", which would be the Anglicisation of Italian Fascismo and those ideologies derived directly from it, such as Spanish Falangism.]
To address the central content of the question, I would say that this is simply a product of the fact that a significant number of authoritarian regimes existed in the twentieth century that claimed some ideological descent from Marx- sometimes tenuously, and for blatantly geopolitical reasons, as in the case of Fidel "Never Finished
The Communist Manifesto" Castro's regime- and involved themselves in the repression and, on occasion, atrocities which you expect from such regimes. If you're asking why so many of these regimes aligned themselves with Marxism-Leninism, then I'm afraid I can't give you a comprehensive answer, but I can detail two major points: firstly, the constitution of the Soviet Union as a class society, and, secondly, the role of Marxism-Leninism in anti-colonial struggles.
Firstly, the USSR as a class society: During the Russian Revolution, the seizure of both land by the peasants and industrial production by the urban workers lead to the overthrow of the old social order which had existed under the Tsar and (albeit already undergoing reform) the Provisional government. However, the collapse of the organs of direct workers power- the soviets (municipal assemblies) and the zavkom and fabkom (factory committees in heavy and medium/light manufacturing, respectively) mean that economic control was increasingly placed in the hands of Bolshevik party-state functionaries (who were usually controlled only indirectly by the workers, if at all), preventing the reconstitution of society in a truly democratic fashion, which is to say, towards communism. (Whether this was a necessity of the war or a Bolshevik grab at power is a venerable debate that does not re-iterating here.) In Marxist terms, what this means is that while the social order was over turned, the basic social relations of society- between capital and labour- were retained, and the social formation was in its most fundamental sense unaltered. (A long, detailed, and boring-to-anyone-who-isn't-a-Marxist discussion of the subject of Soviet capitalism in economic terms can be found
here, for those who wish to at least give it a browse and confirm that I'm not pulling
all of this out of my ass.)
The sum of all this was that the Soviet Union was left in a sort of "manager's socialism", neither capitalism of the traditional privately-managed sort, but not a true "workers' socialism"; all the more so after then New Economic Plan was introduced, which Lenin himself described as a mere "state capitalism" and a step backwards onto firmer ground in wait for the belated international revolution. This left the Bolsheviks in an awkward political position, pushing neither forward towards revolutionary communism, or backwards towards a more traditional capitalism (a move which would have resembled the Old Bolshevik demand for "radical democratic republics" preceding the actual overthrow of capitalism), which, as you may expect, generated certain fractures within the party. Skipping over the sordid details of these intra-party struggles, it is sufficient to say that by the late 1920s, the Stalin party, a faction rooted in the emergent bureaucratic elite of the Bolshevik party-state apparatus, the "nomenklatura", had emerged as the dominant social force. This was in practice a settlement in favour of capitalism, but capitalism of a state-centred form- a purer "state capitalism" than Lenin's conception of the NEP- in which the bureaucratic elite acted as a collective capitalist class, a stock-holding board of directors in a corporation-state (or, in practice, a cartel of corporations extremely heavily tied to the state). Thus, the bourgeois state was revived under a red flag, under a heavy bureaucracy, and under a fetishistic image of "central planning" that, in practice, was no more of a departure from capitalism than the wartime quasi-planned economies adopted by the Western states during the Second World War. Combined with a level of centralised authority originally accumulated in a quasi-Jacobin manner by the Bolshevik government of the civil period, and furthered and heightened under Stalin, class rule became a violent reality to a far greater extent in the Soviet than anywhere else in the Western world outside of the fascist states, as the regime both accumulated capital- through the seizure of peasant lands and the rapid process of industrialisation- and suppressed working class resistance.
All this meant that the Soviet Union was passing on to its political imitators- and not least those which it went about establishing itself- a political order that was still essentially capitalistic, still essentially class-based, and so in which a state power, now unaccountable to the masses, suppressed them and all their attempts to depart from state power, whether towards more market-orientated ends (as in the suppression of the 1954 Hungarian Uprising) or towards more radical, communist-democratic ends (as in the suppression of various anti-Maoist workers councils and communes during the Cultural Revolution). When this is realised, the fact that these regimes varied from a vaguely benevolent oligarchy at best (as in Yugoslavia) to a raging terrorist-state (as in Cambodia) is not actually contradictory with the Marxist understanding of socialism or communism.
However, that only accounts for the existence of such regimes in the fUSSR, and in those areas under its direct influence (Mongolia, North Korea, and Central/Eastern Europe), but not to many of the independently or partially-independently established regimes across the globe. Hence the second point, anti-colonial struggles.
Put simply (and hopefully more briefly), this comes down to the fact that in many countries, most notably China and Vietnam, the Communists were consistently able to pose themselves as the more authentic force of national liberation than the Nationalists (in the general sense of pro-capitalist nationalists; the opposing sides found their classic expression in the CPC-KMT war). This is largely due to the class composition of the Communist movements, which is to say, primarily based in the peasantry, and with a leadership comprised mostly of middle-class intellectuals (although often with relatively few of the urban working class who were, supposedly, their primary revolutionary subject), which allowed them to remove themselves from the web of colonial capitalism that the Nationalists, finding their most significant support among the native capitalists, were trapped in this. This was in part because they were not innately embedded in old-style capitalism, but also because they naturally aligned themselves with the USSR (or in some later cases, with China
against the USSR), which in most of these colonial areas had surprisingly limited neo-colonial ambition, instead seeking the achievement of geopolitical ends which it was willing to pay very good money to see achieved, granting the Communists a not insignificant degree of operational independence. This effectively allowed the Communists to pose themselves as a movement "of the people", embodying the popular and thus national will, in opposition to the "elitist" Nationalists, who often favoured collaboration with the West, if not turning to effective compradorism. (However, it should be noted that there were "middle-grounds", such as Nasser's "Arab socialism", which entailed a highly-interventionist state, but one which did not purport to dictate all economic activity, and largely left those areas outside of the "commanding heights of industry" in private hands; the triumph of any given Communist Party- indeed, its emergence as a major political force- is tied to the specific history of that country, which obviously cannot be elaborated on here.)
And even that is insufficient to give a full explanation, but, as I'm sure the length of this post reflects, that's in a large part because of the complexity of the topic, and so the difficulty of actually giving
fully comprehensive answers to certain questions, so I hope that you'll forgive on that.
Edit: And I see that while I was up to that, everyone else has been at each others throats. Ho hum.
