All the above scenes rely on special effects (bar Divine in Pink Flamingoes).
I defy anyone to sit through the turtle scene in Cannibal Holocaust without wondering why or should they be watching.
Needless to say ,the scene (and many others of equal revulsion) are missing from the UK R2 release of the film.
Outside of fictional stories (albeit with real animal killing) the most revolting footage could be any one of numerous scenes of real death in the Traces of Death series (not to be confused with the fake scenes in Faces of Death).
I'm a hardened gorehound from the days when Fulci films were escorted out the country by the censors,but the Traces of Death series marks the first time ever I have had to switch off partway through,not because I was going to be sick but for its mixture of revulsion and offensiveness