I am not an M-L, Tankie, Stalinist, whatever. I'm of the view that authoritarian forces from the left, especially when acting on behalf of the working class, should be treated with suspicion if not outright opposition. Despite what I think of their politics personally I still try and see from their point of view (it doesn't mean I am adopting it as my own), how they operated, why they operated in such ways, and the local and wider contexts which informed their decisions.
Marxist-Leninists have been explicit in their authoritarianism, and have had no tolerance for any popular political tendency they can't control for their own ends. They have had particular frameworks, devised by the USSR, either rigidly applied or local innovative adaptations, through which to understand the reality of their particular situations and then act upon them. Taking over, neutralising or destroying the organisations of political rivals was a part of that. In partitioned Vietnam's case they did have to be ruthless, for their very survival. Whether or not you think they deserved to survive is neither here nor there. Within the Marxist-Leninist political tradition of the ICP, and using both the Soviet and Chinese doctrines internalised and adapted to local conditions, they acted as they saw fit to ensure their survival, and then seek to win political control of the country from their enemies.
Kev brought up the USSR firstly, then I used a related political movement in another country to illustrate the compromising realities of revolution, where eventually my violence has to defeat their violence. The horrific actions of the Vietnamese I used weren't born out of nothing, they aren't unique to Communists, although how they understood them may differ. At one point in a revolution where anarchists really do have to fight to survive from attacks by the authoritarian forces of the left and right, does a firing squad become a justifiable course of action? When do several killings in hasty circumstances become a terrible necessity? When does something done in a situation of shifting or skewed senses of morality become excessive?