Taking just Cambodia - the Khmer Rouge regime was of course supported by China with massive resources. However, the US has also killed tens of thousands of Cmabodians directly in bombing campaigns during the Vietnam War, between 30,000 and 150,000 (likely committing the war crime of Aggression, as they were not in a state of war with Cambodia); this helped galvanize the population towards joining the enemies of the US, helping the Khmer Rouge attain power.
Later on, in 1979 when the re-united Vietnam attacked the Khmer Rouge, both the US and China condemned the actions of the Vietnam liberators, and helped defend the Khmer Rouge from international opinion (since Vietnam was being supported in this by the USSR). The US sent lawyers to defend Pol Pot himself from the tribunals in the newly liberated Cambodia, and both China and the US helped keep Khmer Rouge affiliates on the UN seat for Cambodia until 1993. Of course, China went further and launched a retaliatory war against Vietnam for their liberation of Cambodia.
You also seem to be greatly minimizing the US involvement in many of these places. The US occupied Nicaragua for 20 years, installed a military dictatorship that ruled it for 50 years, and then trained, armed and directed guerillas that killed and tortured hundreds of thousands of civilians to try to stop the revolution that took down the Somoza dictatorship. Much of the rest of South America saw similar treatment from the US: coups taking down socialist popular movements and installing military dictatorships instead, then supporting violent militias if counter-revolutions started.
In the middle east, while many of the problems started with the British handling of the collapse of the Ottoman empire, and the USSR also led brutal wars of oppression, the US has picked up the torch in the last 30 or so years as the major source of strife in the region. They are responsible for:
- arming and training the Taliban
- installing Saddam Hussein in power, arming him, and directing him to attack Iran, even accepting his use of chemical warfare
- squashing Iran's brief democracy under Mossadegh, installing a military dictatorship under the Shah, starting the Iran Nuclear Weapons program, arming them (and then losing control to the islamic revolution)
- funding and arming Saudi Arabia and allowing it to spread its demented form of islamic fundamentalism, supporting even as we speak their invasion of Yemen
- supporting the Israeli apartheid state, illegal nuclear weapons arsenal, and illegal occupation of Palestinian territories
- invading Afghanistan in 2001 and killing thousands in the process, causing the deaths of as many as 100,000 civilians overall in the conflict