I think the communists also frequently exploit real injustices. For example, in South Vietnam, the US kept urging the government there to implement land reform, also a communist cause.
In Indonesia during Suharto's time, I wonder what injustices, like feudal land arrangements perhaps, the communists exploited.
It was actually ‘land reform’ (as I recall many liberals called Ho Chi Minh an “agricultural reformist”, so much nicer than “Communist” don’t you think?) that caused the problems for the Communists in the first place.
Thinking they were on the verge of victory under Sukarno’s rule they started “actions” in the villages, seizing the lands belonging to “landlords” or “capitalists”, in effect anyone with a half decent landholding saw their property being stolen by cadres sent down from Jakarta. Very quickly people began to resist the Communists, by 1965 there were extensive networks of anti-Communist resistance fighting the land grabs by the Reds.
Following the abortive coup of September 1965 the Army under Suharto was able to mobilise these anti-Communist forces and the Communists were utterly routed.
It was not the army but local people who exterminated the Communists, local people very unhappy about having their land stolen by political apparatchiks.