Conversation so far:
When was the Khmer Krom separatist movement?
The Khmer Krom separatist movements was between 1954 and 1976.
What was special about the Khmer Krom separatist movements?
The anti-Communist prime minister of the Khmer Republic (1970 - 1975) Lon Nol planned to recapture the Mekong Delta from South Vietnam.
Was the Khmer Krom separatist moment helpful?
67 Khmer Krom fighters were all massacred.
Are there any other interesting aspects about the Khmer Krom separatists movements?
The White Scarves arrived in Kiri Vong District in 1976, making overture to the Khmer Rouge and appealing to the leader Khieu Samphan directly for assistance.
Were any others killed in the Khmer Krom separatist movements?
Subsequent orders from the Khmer Rouge leadership however had Samouk Sen arrested, taken to Phnom Penh, tortured, and killed. His force of 67 Khmer Krom fighters were all massacred.

Question: why were they killed?
Self-contained rewrite: Why were they killed?