I think you are confused with regards to the terminology.

Initially, the terms "Socialism" and "Communism" were somewhat interchangeable, with different shades of meaning applied by different intellectuals. The first concrete definition of Socialism was given by Vladimir Lenin after the Russian Revolution of 1917, referring to a stage between Capitalism and Communism; and for Lenin that signified specific series of reforms geared towards modernizing Russia's economy, since many believed it was too backwards to make a true Communist Revolution possible. In Lenin's Socialism, small businesses and agriculture continued to be privately run, while banks, big businesses, key industries, and foreign trade were handled by the state. Of course, Lenin died in 1924 and a debate began on how economic policy should continue, as the industrialization rate the previous policy, although improved, had been slow and unsatisfactory. Joseph Stalin, the General Secretary of the Communist Party, won out in the end. Stalin nationalized what amounted to the whole of the economy, and being an engineer by training ran Russia with specific directives stipulating, "If we input this much X, we should get this much Y."

Was a Stalinian dictatorship inevitable in Russia given the post-Revolution (and Post-WWI) conditions? I don't know. Farm to Factory: A Reinterpretation of the Soviet Industrial Revolution by Robert C. Allen (Princeton University Press, 2003) is an acclaimed text that gives a thorough overview of the Soviet decision-making process and how it changed as the Communist Party changed. It might be interesting for you to look at it.

On the other hand, in Western Europe, there is a definite split between Communist Parties and Sociality Parties in the very early 1920s. Generally (although it varies country by country and party by party) following the third Communist International Meeting in Moscow, Lenin stipulated that to call itself a "Communist Party" and to be a member of the Communist International, a party had to be geared towards agitating for a global communist revolution. In many countries leaders thought this was a little too much, so the left was split between "Socialists" advocating for legislative reforms, and "Communists" advocating for revolution.

As a consequence of this, most left-wing parties in Europe and in the world are deeply rooted in 1920s socialism. In France, for example, the Parti Socialiste is the principal left-wing party and the party of the current President, François Hollande; and France is hardly on the brink of a communist revolution, in fact the current Presidential campaign would indicate quite the opposite! So there are countless examples where socialist and socialist-inspired policies have been implemented without "turning to communism." In fact, the most important French president of modern times, Charles de Gaulle, in spite of being a unique nationalist independent with no clear political definition, was in many ways aligned with the Socialists in his economic policy which embraced Dirigisme, a concept where the State invests in or outright owns important industries.

The most complete manifestation of the left-wing political spectrum is in Italy between 1947-1992, although you can certainly argue that the Communist Party stops being recognizably Communist after their leader Enrico Berlinguer's "Historic Compromise" in the 1970s, giving rise to a "Radical Party" even farther to the left in addition to a number of extra-parliamentary revolutionary groups. Regardless, in Italy there was a large Communist Party on the far left, a Socialist Party representing the "vanilla" left, and Social Democratic Party representing the center-left. The Christian Democracy Party, which won a plurality in every single election between 1947 and 1992, multiple times resorted to coalitions with the Socialists, including one famous coalition between 1982 and 1987 under a Socialist Prime Minister; Bettino Craxi. Following that coalition, the Communist Party collapsed due to external as well as internal factors, so you could say the exact opposite of your question premise: a Socialist Prime Minister helped bring about the end of Communism in Italy!