Complex sentence: He was together with his boss Duch seeing to the execution of surviving prisoners before abandoning Tuol Sleng prison and were among the very last Khmer Rouge cadres to flee Phnom Penh when it fell to the People's Army of Vietnam on 7 January 1979.
Split into simpler sentences: He was together with his boss Duch seeing to the execution of surviving prisoners before abandoning Tuol Sleng prison. They were among the last Khmer Rouge cadres to flee Phnom Penh when it fell to the People's Army of Vietnam on 7 January 1979.