Memes

Marx wasn't the only socialist of his time, though history has deemed him certainly the most important.

Lenin on the other hand...

Nothing about the theory required Lenin to overthrow the results of a democratic election in 1917

Nothing about the theory required Lenin to advocate for the purging of his other fellow socialists in the soviets/councils.

Nothing about the theory required Lenin to backstab and crush his anarchist allies in Ukraine.

Nothing about the theory required Lenin crushing anarchism/syndicalism in Russia.

Nothing about the theory required Lenin to crush the working class when they told him outright that his actions were against their will.

Nothing about the theory required Lenin banning all dissent even within the Communist party.

Nothing about the theory required Lenin to start mass seizures of food and mass nationalizations under "War Communism" that started the canard of "Socialism is when the government owns things".

By the time of Lenins death, he had put in place all the institutions and levers of control that Stalin would later use to brutalize the population, all the worst parts of the Great Purge can be connected back to the systems that Lenin put in place.

Lenin had immense opportunities for positive change as the leader of the first socialist nation, however he squandered it completely by his purge of any ideals that weren't his own, and his project turned into a dictatorship. Meanwhile the Democratic Socialists and Anarchists that Lenin loved to berate helped build societies that are now infinitely better to live in and more open to change from the working class than any of the modern countries inspired by his ideology.

You can say "Marx is the theory , Lenin is the practice" but it's much more accurate to say, Marx is the theory, Lenin is a practice, and not the best one historically.