I think there is a difference between downloading a video because it happens to be in front of you and because of curiosity and being disgusted afterwards; and actively seeking such videos on the internet or on black market DVDs or whatever because you enjoy to look at such things.
It has been mentioned earlier here that MANY people tend to slow down to look at a car crash. You know it's going to be nasty, you know what has happened and you've seen accidents before but still you "want" to see.
I don't know myself what pushes people do look, morbid curiosity, adrenaline rush because it scares you, maybe the same as a scary movie but scarier because it's real - the fact is people look if it's easily available, doesn't mean they like what they see.
Movies and documentaries about the horrors of concentration camps during WWII are on TV day in day out and some are pretty graphic yet we watch them for our "education" and because it's part of "history". Does it make a difference watching half (or completely) dead people from WWII or from 2 months ago in Iraq ?