If you listen to the lyrics, the girl takes him to meet her family, they hold out their guns so he can see them and try to speak to him (not how I would conduct an ambush), but since he doesn't understand them he grabs the little sister and uses her as a human shield. They take this as an act of aggression and a firefight starts, during whch the little girl gets shot (doesn't say by whom but if the blood sprayed out from between her eyes it sounds like she was shot in the back of the head) and then the marine kills the father and brother.

In any case, there is a lot of laughter associated with all this killing, which I find inhuman. You are free to take it any way you choose, but it makes me sick.