When i say I've never seen a real snuff scene of someone being beaten or stabbed to death, it's with the exception of heavily choreographed scenes (Errol Flynn, Bruce Lee etc) that don't try to look truly realistic and gritty. There is a scene in the movie The Passenger (1975), starring Jack Nicholson, of a man being bound to some gas barrels and shot by African guerillas; I've seen it. That one is supposed to be genuine footage of a real execution, it's unclear whether the film team took it themselves or they were handed it by someone but they were filming in a part of West Africa where guerilla warfare was going on, and it fitted into the story.
I did not see The Passion of the Christ.