"Cambodia's genocide was Nixon's fault!"
I didn't say that.
I did say however, that Nixon's involvement in Cambodia slid a neutral nation into complete chaos by large scale bombing.
Perhaps you should study what happens to a nation when it gets bombed heavily. Societal cohesion is not one of its' main hallmarks.
But the nation of Cambodia as a whole was not subjected to large-scale bombing. The bombing was along the border with South Vietnam, a region that was controlled by North Vietnam and not the Cambodian government. How could that have caused "complete chaos" in the rest of Cambodia?