Content warning for gore.
The Act of Killing follows the notorious executioners of the Indonesian Massacres of 1965-66, in which the Indonesian government instigated violence against known communists & leftists, and the Chinese population. The filmmakers cooperate with the "gangsters" who commonly carried out the orders of the Indonesian government in an attempt to recreate the massacre fifty years later.
You'll find, when you watch this film, your inability to be absorbed by the documentary, but it's intentional. It's meant so that each scene is jarring, so that the viewer is always aware and always critical. You're not supposed to be lulled by the narrative. As summed in my one-sentence review on Letterboxd:
there is nothing hidden to this film.
There are some historical contexts the movie doesn't provide. This documentary explores Indonesia's cruelty toward communists and the Chinese, however doesn't address that in a deal made with the United States for annexation of West Papua, Indonesia was implored by the US to reduce the communist influence in Indonesia. We might have lost Vietnam, but we succeeded in devastating Southeast Asia.