I really don't think true snuff films exist. If I remember correctly in order to qualify as a true snuff film, the actress/actor has to come in thinking they're starring in a porno or horror film and then find out after the fatal blow that they've been deceived. There has never been a documented case of this happening. Like Siorai said, there's no reason to assume this couldn't happen, but I don't think it has.
As for the argument that making violent films leads to a progression to commit violent acts, that is hogwash. Violent horror films and video games have always made very convenient scapegoats for real life crimes, but the fact of the matter is atrocities have been commited by sick individuals since the dawn of man and they needed no movie as a trigger. I don't think sitting in your room watching Saw or August Underground for days at a time is going to turn you into a violent person (it might give you quite a headache though).
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