After 1979's Vietnamese liberation of Cambodia, major foreign governments such as the UK and USA supported the Khmer Rouge dominated CGDK holding Cambodia's seat in the UN from 1990-1993, and also seemed to provide support to the Khmer Rouge at various other times after they were largely removed from power in 1979. What drove this support for the communist perpetrators of the Cambodian Genocide?
Were people unaware of the extent of the Khmer Rouge's crimes due to lack of information? Were they mistrustful of the information from the liberating communist Vietnamese? Were other political goals seen as worth supporting the group for?
Were there any other motives for supporting the Khmer Rouge?
Support of the Khmer Rouge from 'the west', was largely the result of Cold War realpolitik. Vietnam, aligned with the Soviet side of the Sino-Soviet Split, represented an unwelcome possibility for further Soviet influence of the Indochinese region. China, Democratic Kampuchea's biggest ally, sought to keep the Khmer Rouge as a 'thorn in the side' of the new PRK Government... (read Vietnamese Government in Cambodia).
US and UK support for the Khmer Rouge in the UN came from an unwillingness to acknowledge the legitimacy of the Vietnamese invasion of Cambodia, an attitude based in part on the US's recent abdication from their conflict in the region and the US/Chinese rapprochement.
Please see these answers for slightly more information Why did the US back the Khmer Rouge if they were communist? and How come the US and British Government supported the Khmer Rouge?
They both answer the main thrust of your question, 'what drove support for the Khmer Rouge'. But the other questions you raise, like 'were people unaware', 'were they mistrustful' are interesting.. I don't have time to give you the answer these really deserve but, I will give something brief.
Were people (governments) aware? Yes. The CIA knew what was going on in Cambodia for years. An indication of how much the US knew about the human rights record in question can be seen in the reaction of a member the US delegation, Robert Rosenstock, when after being thanked by a senior Khmer Rouge member (Ieng Sary) after the vote to keep the KR in the UN was passed, he said 'I felt like washing my hands.' I realise that seems slightly apocryphal, but to me it says a lot. Professor David Chandler, in an interview with me, stated that it was during the final year of the regime that the consensus on refugee reports went from 'can't tell' to something terrible had been happening.
Were people mistrustful of the Vietnamese? Yeah, sure. With good reason too. It is a sticky subject to get into but they did invade Cambodia. Yes, most people agree that it was in defence and warranted, but to the extent that this invasion was designed to 'save' Cambodia.. that is debatable. Did it have that effect? Sure - was it the intent? ... no. Vietnamese propaganda following after the CPK had been ousted was focused on painting the picture of a Nazi-esque genocide based on race, which isn't the most applicable framework to place on the Khmer Rouge period at all.
All of that isn't very relevant to your question though.
To circle back slightly, we are still talking about significant financial support from the 'west' for a group that had a well known and extremely troubling human rights record. They knew too well what they were doing, and to me it ranks highly in the most cynical... depressing and callous manoeuvres of the Cold War. Costing many innocent lives and crippling Cambodia's economy for years.