I believe what has happened is the young Marines opened themselves up and trusted this guy and, probably assumed that he was their good buddy. Little did they know what a snake he really is.
I recall a fable about some animal or frog, that was convinced by, I think a poisonous spider, or maybe a snake to allow it to ride on its back across a river, but halfway across it bit the animal and as the animal died, he asked why it bit him. And the reply was something like, "But that is what I do."
"he asked why "
could also have been the gingerbread man and the fox? must be a common theme . . .