There are two general 'forces' that can be used to explain the rise of Pol Pot (Saloth Sar) in Cambodia and the Khmer Rouge revolution in 1975. I would identify these forces generally as; the internal, meaning aspects of Cambodian history and culture that played a role in facilitating the conditions needed for a revolution to take place and the second being the external, meaning the elements of global history at play which in conjunction with the internal factors led to the devastation of the Khmer Rouge regime. Perhaps a good metaphor for this would be a hurricane (or cyclone/typhoon depending on where you live) where both 'high' and 'low' pressures combine to create an intense storm that would not be possible without these two elements.

So this answer will seek to explain the relationship of these two aspects, the 'high and low pressure systems'.

The second part of your question, relating to the justification of 'the Cambodian genocide', as this is a long enough answer as it is, can be found addressed in my answers about 'was the Khmer Rouge period a 'unique form of genocide' and what were the reasons for the anti-intellectual killings of the khmer rouge , the aspects of theravada Buddhism related to the Khmer Rouge period as well as the definition of genocide and its applicability to Cambodia/The Khmer Rouge here

Sources used for the answers below are Philip Short's biography of Pol Pot (history of a nightmare), David Chandler's History of Cambodia, and a Political Biography of Pol Pot, Ben Kiernan's The Pol Pot Regime as well as How Pol Pot Came to Power. This is a long read so if you are after a shorter version you can check out a few different resources like the documentary 'year zero' by John Pilger, or this once from the BBC in the late 60's (before Sihanouk was ousted) and for a kind of audio-version of this answer you can check out my podcast 'In the Shadows of Utopia' on all podcast servers or on youtube here