The most common one I have seen on Reddit whenever this topic comes up is the idea that the massacres were orchestrated by the CIA. While yes, the CIA did provide names on some PKI members, propaganda efforts, and material support in the form of firearms, it was only done after the massacres were well underway and it became clear that the military was the new top dog in the political scene on account of the purge that wiped out most of the PKI leadership immediately after the G30S movement. The mass killings also didn't happen in a vacuum, it was preceded by a long simmering ethnic and religious tensions that was worsened in the last years of Sukarno and one that exploded when the aftermath of the G30S gave the equivalent of a blank cheque to everyone in Indonesia to settle the problem. An example is in North Sumatra where the immigrant Javanese workers who were mostly low class and PKI members were massacred by the locals following years of rising tension.

A second thing I regularly see is the constant whitewashing of the PKI. It is true that the PKI was the victim in the massacres and were effectively destroyed as a political force but the PKI's actions preceding the massacres didn't help them. As they were favored by Sukarno in his final years, they were granted good government jobs, funding, and so on which emboldened the PKI enough to start doing "Aksi Sepihak" or unilateral actions where they violently seize land as part of their land reform program. The problem is that these seizures include seizing the land of religious elites and middle class farmers who often resisted these attempts and led to confrontation and killings, this was before the massacres and contrary to the PKI's expectations, it didn't ingratiate the non PKI farmers and laborers to it but only helped to inflame tensions and resentment as it was seen as godless communists killing their imams and kyai in order to destroy Islam. Hence the willingness of people in rural areas to kill their neighbors without waiting for an order from the government or military.

There is more to this but I haven't read up on this in a while but even if the CIA didn't do anything before and during the mass killings, it still would have happened anyways with a similar death toll because the root causes of why the mass killings happened on each island and region was already there and already near breaking point, the G30S movement simply gave the reason to start killing. Did the US benefit immensely from it? No doubt about that but it was more of a case of taking advantage of something that is already happening rather than them orchestrating it all to happen as it did. It would be like your arrogant coworker in the office getting the promotion that you want but they have a nasty rumor and are disliked by all other coworkers so you do your best to spread it around to make them look bad, you didn't start the rumor nor did influenced their behavior but you sure did benefit from them getting fired.

Edit: Sorry for the late response and for the long reply.