Various torture murders are probably the most horrific things I've read about:
Junko Furuta, who was a perfectly normal girl until she was kidnapped off the street by a gang for no reason (purely wrong place, wrong time, according to Japanese sources) and unimaginably raped and tortured to death over 44 days.
Kiki Camarena, a DEA agent in Mexico who was kidnapped by the Guadalajara Cartel and tortured to death over about three days. His story is the central focus of the show Narcos: Mexico, but from what I've read, his death was even more brutal than what they make it out to be. A detail consistent in both is that they brought in a doctor to revive him numerous times when he got too close to death, and keep him conscious so he couldn't pass out from the pain.
Kelly Anne Bates, who was groomed from a young age by a scumbag who eventually killed her slowly over the course of about a month in their apartment. Her injuries were so horrific the jury at the guy's trial had to get counseling. For example, her eyes were missing and there were stab wounds in the sockets.
Sylvia Likens, whose parents left her in the care of a woman who both personally and with the help of her children and their friends degraded, tortured, and killed over over several months.
The Hello Kitty murder in China, where a man and his about-13-year-old girlfriend kidnapped a prostitute who had supposedly stolen from him (even though sources say she had paid him back with interest), tortured her to death, and then stuffed her corpse into a Hello Kitty toy.
Baby Brianna, a newborn whose mother, father, and uncle tortured and raped her from the second they brought her home from the hospital until the second she died.
Baby P, a British toddler whose mother let her boyfriend torture him to death. His back was broken in his last days of life.
Honestly, there are just so many horrifically on par with each other, you could list them all day.
