In the 19th and early 20th century social democracy = Marxism. The theory of German social democracy includes Marx and Engels. Lenin was a social democrat who belonged to the Russian social democratic labour party and wrote texts about what social democrats should do.

In this period the term communism was generally used by anarchists and not Marxists. This changed when the majority of social democratic parties supported WW1, unlike the majority of the anarchist and syndicalist movement which opposed it.

In response to the betrayal of the social democratic movement Lenin had an absolutely massive existential crisis and decided that the war should be transformed into a revolution, which historically was an anarchist position and had been attempted unsuccessfully by Bakunin in 1870

As part of the break with the social democratic movement Lenin and other Bolsheviks renamed their party the Russian communist party and founded the communist third international in 1919 which opposed the social democratic 2nd international.

In 1917 Lenin wrote state and revolution. It was an attempt to return to the original theory of transition that had been advocated by the main founders of German social democracy - Marx and Engels - but abandoned or distorted by the subsequent movement.

In this sense Lenin never stopped being a social democrat even though he adopted the label of communist in order to differentiate himself from the actual social democratic parties who had betrayed their original principles.

State and revolution was never implemented by the Bolshevik party and they in fact did the exact opposite of what it advocated on a number of occasions. This occured before the civil war had even begun. https://itsgoingdown.org/bolshevik-myth-reloaded/