Hisoka tied with razor-sharp wire to an upright bed, while being forced to remember the night Muraki raped him.
Sangatanus feeling-up "Hijiri" ( actually Hisoka in disguise), then brutally murdering him. ( because it was Hisoka, a Shinigami, he got better.) That entire plotline was massive Paranoia Fuel. Getting an organ transplant? Not usually A Fate Worse Than Death. Having said organ come from someone who made a Deal with the Devil, and now said devil has declared you fair game? Not good at all.
Kazusa's death. We don't see it happen, (though the manga shows us the—thankfully not graphically detailed—results) but we do know that she pushes Hijiri out of the way of a collapsing pillar, meaning that she was crushed to death underneath it. She gets better, but still. Oh, and did we mention that she appears to be eight years old?
The St. Michael's School arc, which was so horribly full of Nightmare Fuel and Squick that the author made the next arc as an apology.
The reason Muraki obsessively stalks Tsuzuki? He wants to put his brother's head on Tsuzuki's body, bring Saki to life, and kill him.
Muraki in general. Murderer, rapist, driving everyone around him insane, manipulative, Magnificent Bastard, a doctor (which, yes, should terrify you) and many who see him (he dresses in white, is pale, has pale eyes and hair) think of him as an angel.
Actually... It's worse than that. Nagare didn't make a deal with Yatonokami. His ancestor wasn't strong enough to kill the god, and instead imprisoned it in his own body. To keep it from escaping and murdering everyone in the village the god has been passed down from father to son for generations, each clan head bearing it - and taking the brunt of the rape. This is what would have awaited Hisoka, had he not been raped and killed. Woooooow.
And let's not forget that Hisoka's mother was forcibly impregnated by the same demon that constantly rapes Nagare, and Rui's pregnancy with Gods-know-what kind of hellspawn has lasted for at least two years. In story.
It's not a demon, it's a god. Whether that makes it better or worse is up to you.