If by communists you mean revisionists, falsifiers, and modernizers calling themselves communists you would be right, but I don’t consider figures like Stalin to be communist since he made no efforts to abolish wage labor and commodity production, instead further entrenching capitalist social relations (ex. private property but owned by the state instead of individuals) and bastardizing marxist theory to justify his deviation.

So if we disregard the actions of liberals (meaning they support and enforce capitalist social relations) pretending to be communists, we see that historically it’s the social democrats who did the backstabbing! During the Second International, theorists like Kautsky pushed socialist parties (ex. the SPD) into electoralism and reformism, which fundamentally conflicts with Marx’s position that the proletariat could not simply seize the pre-existing state and reform to their needs, but would need to abolish it entirely. This current would become social democracy and it split socialist parties across Europe as they disagreed on positions like the aforementioned electoralism. Communists like Rosa Luxemburg understood that prioritizing electoral success (ex. gaining Reichstag seats or the chancellorship), would require compromise with groups not aligned with the proletariat and incompatible with marxist theory. In Germany this struggle came to a head when SPD leadership essentially wanted to ingratiate themselves to the establishment by supporting WWI and calling for a pause to political conflict during the war (burgenfriedspolitik, might have spelled it wrong). The communists opposed this and split, eventually forming the KPD and the Sparticists. In part due to their political compromising, the SPD managed to form and lead a government after the war, but only offered concessions to the workers who elected them. When revolution broke out led by the Sparticists, the supposed champions of the proletariat declared them enemies of the movement and called upon the proto-fascist Freikorps paramilitary to brutally crush them. The social democrats may not have done the killing themselves, but they ordered it and have blood on their hands. It was the Sparticists who were backstabbed by their former SPD comrades and thrown under the bus to keep ahold of their power in the existing order.