I see this as similar question to "How can someone watch extremely violent movies, when I'm only enjoying kids friendly cartoons?"
There will always be some level of preference towards something which you never can fully explain.

With hard fetish content, I can't say as I don't have any and I'm not professional, but there are couple aspects that should be easy to grasp on. Many hard fetishes are taboo, they are something that by popular norms have been deemed as something that are wrong and shouldn't be done. Snuff, gore, death, etc. you are at the pinnacle of that, but then there is also that feeling that you will (hopefully) never get to experience in real life, playing with someone elses life, having that level of control over it that you can also choose to end it. Similarly to something like asphyxiation, the feeling of you going near death - or dying - would give some immense adrenaline rush on itself if nothing else.

Of course because of the nature of it, it also does have extreme amount of shocking value and I know many who enjoy getting those reactions out of everyone. (See: Munkelzahns avatar)

And I guess that's what this whole site kinda boils down to with content under explicit rating. All everything is still fantasy, many things here are things that nobody will never want to do in real life, but still want to indulge into feeling of it.