I'm reading about the Khmer Rouge and communism in Cambodia. In the Khmer Rouge's ideology, violence isn't just encourage but extreme acts of violence are seen as an essential part of having the proper 'revolutionary consciousness'.
Obviously the Khmer Rouge took those ideas to a extreme. And while their particular style of communism was very much outside of the norm, they were heavily influenced by Stalin and Mao.
What I'm curious about, outside of the Khmer Rouge, is how communist thought on political violence and party-purges evolved from Marx to Mao. In the communist theory of the time, how much was violence essentially encouraged or thought to be inevitable, how was it justified, and how did it evolve?
I'm not a historian myself but I did go to school for political theory and I have extensively studied Socialist theory so I can touch on some of the nuances of the theoretical views of different figures.
Marx himself was not involved in any violent purges (although he did get his political opponents, the anarchists, kicked out of the 1st Internationale), however he understood that violence would be necessary for the working class to overthrow the bourgeoisie. To be more specific, in the context of the 1848 revolutions sweeping Europe, Marx called for the creation of armed militias and maintained that access to weapons was necessary for the workers to fight the bourgeoisie:
To be able forcefully and threateningly to oppose this party, whose betrayal of the workers will begin with the very first hour of victory, the workers must be armed and organized. The whole proletariat must be armed at once with muskets, rifles, cannon and ammunition, and the revival of the old-style citizens’ militia, directed against the workers, must be opposed. Where the formation of this militia cannot be prevented, the workers must try to organize themselves independently as a proletarian guard, with elected leaders and with their own elected general staff; they must try to place themselves not under the orders of the state authority but of the revolutionary local councils set up by the workers. Where the workers are employed by the state, they must arm and organize themselves into special corps with elected leaders, or as a part of the proletarian guard. Under no pretext should arms and ammunition be surrendered; any attempt to disarm the workers must be frustrated, by force if necessary.
Marx and Engels were proponents of revolution and acknowledged that it would be a violent event where the working class imposed itself onto the bourgeoisie:
A revolution is certainly the most authoritarian thing there is; it is the act whereby one part of the population imposes its will upon the other part by means of rifles, bayonets and cannon — authoritarian means, if such there be at all; and if the victorious party does not want to have fought in vain, it must maintain this rule by means of the terror which its arms inspire in the reactionists. Would the Paris Commune have lasted a single day if it had not made use of this authority of the armed people against the bourgeois? Should we not, on the contrary, reproach it for not having used it freely enough?
Marx did not believe the revolution would be one single violent event but would culminate in a dictatorship of the proletariat in which the revolutionary movement would use its newfound position of power to violently suppress the bourgeois elements in society. Marx was a firm advocate for revolutionary terror, not unlike the revolutionary terror inflicted by the Jacobins when they took power during the French Revolution:
The purposeless massacres perpetrated since the June and October events, the tedious offering of sacrifices since February and March, the very cannibalism of the counterrevolution will convince the nations that there is only one way in which the murderous death agonies of the old society and the bloody birth throes of the new society can be shortened, simplified and concentrated, and that way is revolutionary terror... We have no compassion and we ask no compassion from you. When our turn comes, we shall not make excuses for the terror.
That being said, the focus on revolutionary violence is not universal among Marxists, there were so-called "reformists" who believed that the transition from capitalism to socialism could be achieved through social reforms instead of a violent revolution. The most prominent example is Eduard Bernstein although it is worth noting that they are generally referred to as "revisionist" Marxists for revising many of Marx's central tenets.
Speaking of dissident Marxists, there is also Georges Sorel who openly praised violence as a beneficial thing that had positive attributes in his text, "Reflections on Violence". It is worth noting that Sorel's worldview was fairly divorced from classical Marxism and his ideas on violence and the myth of revolution ended up having an influence on Fascism.
Although not a Marxist himself, I also want to briefly touch on Sergey Nechayev and his revolutionary outlook and how it affected the thought of later Marxists. Sergey Nechayev is most known for his Revolutionary Catechism (it is pretty short and I recommend giving it a read) where he argues that the ends justify the means to the extent that professional revolutionaries must become cold killers who are willing to sacrifice their own comrades for the sake of the revolutionary cause. This short text has had an influence on more recent Marxist groups such as the Black Panther Party and the Red Brigades, but also influenced the most important revolutionary of the time, Vladimir Lenin.