I think it’s the intention, too. Watching a documentary on the Holocaust means you’ve gotta see the shocking images. The images are supposed to shock you. OTOH, there is also morbid curiosity and I think I try to avoid that. I don’t need to see somebody being beheaded in Iraq and so I won’t seek out the clips. I know someone who did and he said he really regretted it.
The Passion of the Christ is purposely gruesome because it is intended to arouse a certain emotional response. It’s OK to purposely use this if the intended response is OK, but you can be sure some people will watch it for the sheer thrill of it.
Sometimes, I think I “get off” on watching horrible things like holocaust footage and that shames me. No, I don’t mean I cheer when Jews are killed - I mean that I simply get off in some way on the visceral reaction to it.
I remember St. Augustine talked about the possible negative effects the circus could have on people. Aside from the fact that it was simply cheap, idle entertainment that distracted one from what they should be attending to, he also knew that it could really warp a person’s mind. His best friend was dragged to the circus once and he wasn’t able to resist looking… after that, he was unable to look away and he became addicted to the violent spectacle.