You should not be embarrassed by the question - especially due to your own circumstances.

Answering 'why people blame the US', generally speaking, can (I think) be traced back to a series of events or circumstances. Following the first Indochina War, the US had a policy of containing communism and backing non-communist regimes in the region. This will (eventually) lead to the Second Indochina War (or the Vietnam War). The impression most people have of this war is that it was unnecessary for the US to engage in, it was a failure of foreign policy and demonstrated a lack of understanding of the reality of the region it was fighting in. It was also undoubtedly costly in terms of human life. The war spilt over into Cambodia and if you think that the Khmer Rouge were the result of that war then you might blame the US.

However, the idea of, "well if there was no Vietnam War then there would be no Khmer Rouge", is a little over-simplified and ignores how history goes much further back if you are playing that game. Why not blame the French for their inept colonial rule? Why not blame the Japanese for fostering nationalist movements in the region? Why not blame Colonialism... or go even further back to the relationship between the states that would become Indochina in the 19th century? I think it is a silly game to play.

The Cambodian Civil War and the ensuing period of tragedy at the hands of the Khmer Rouge is highly dependent on the context of the era but also the factors put in place decades and even centuries earlier. The role of Prince Sihanouk is important to emphasise, as is the role of international communism. I feel as though we are already wading into the territory you explained in your question as overly complex, and I apologise, but it is a complicated question.

If we get back to simply why the US is often blamed, I think this also comes from ideas prevalent in western culture in the aftermath of the Vietnam War. A disillusionment with US policies, the tremendous cost of life, portrayals of the military in certain popular movies, as well as by prominent left wing thinkers.

The secret bombing of Cambodia, although terrible, is also debatable when it comes to the results of the bombing vis a vis Khmer Rouge recruitment. There is an often quoted KR cadre who confirms that it was very helpful. However, this also kept the KR at bay from conquering the country for years more than what it would have taken them without being bombed. Likewise, one needs to look to the influence of someone like Prince Sihanouk. Once he joined a coalition of groups led by the Khmer Rouge, many civilians in the countryside were convinced to join as well, perhaps far more than those who were recruited due to the carnage resulting from US bombing raids.

I am rushing through this as I just woke up and need to go to work, but there is one final thing I want to mention.

I think the tendency to remove agency from the Cambodians who led the Khmer Rouge is also a mistake. Blaming everything on the US is too simplistic and does not account for the horrific treatment meted out by Cambodians to other Cambodians. The US did not invent S-21.

Perhaps I will be able to return throughout the day to make amendments and edits to tie this all up a bit more.

All the best, and if you are looking for a simplified, yet detailed, explanation of Cambodian history and the Khmer Rouge feel free to check out the podcast I produce, visit www.shadowsofutopia.com for more info