I read somewhere that the VC in the photo had just hand-grendaded a ARVN soldier's family killing his wife and kids.
Of course, that's a minor issue compared to the brutality and heavy-handedness of the South Vietnamese in shooting the scumbag. </Fonda-ism>
The VC in the picture was a regular in the NVA caught behind the lines in civilian clothes performing sabotage. The Police Chief summarily executed him.