While Yu Yu Hakusho's demons weren't to be taken lightly, it's made clear throughout the series that humans could be just as, if not even more, dangerous. This is proven by the Chapter Black tape, a video full of the worst humanity has to offer that was so deplorable, it caused several humans to want to destroy their own kind and team up with Shinobu Sensui to open the gate to Demon World. While Sensui was definitely dangerous, there is one other human who may be even more so: Sakyo.
Sakyo was born to an average family of white-collar workers and despised how they went along with society's rules without a second thought. His family was poor but loving, yet he wasn't happy with his life. He began killing animals and insects to feed his boredom, and in the manga, he even talks about dissecting humans. He was obsessed with death and started working at a pet store where he had access to numerous animals to "experiment" on.
As an adult, Sakyo learned that he could use this mindset to earn money gambling, and eventually he became rich enough to join the exclusive Black Black Club, a group of the wealthiest men on the planet. To secure his influence within the club, Sakyo started a demon slave trade, but that also began to bore him. He channeled his addiction to gambling into betting on the Toguro Brothers in the Dark Tournament. He also began planning to open a tunnel to Demon World big enough for the most powerful demons to enter the Human World. He wanted to watch them use their full powers against humanity without Spirit World interfering.
Sakyo makes a bet with Koenma that, in order to prevent a draw, the owner of the losing team would die, which Koenma accepted. When Toguro began absorbing the souls of the tournament attendees, leading them to try and escape, Sakyo sealed everyone in the arena. He claimed that the tickets to the tournament were worth more than the lives of everyone who bought one. Keeping up his part of the deal with Koenma, Sakyo killed himself in an explosion that destroyed the tournament arena.
Sakyo was the one who initially proposed creating a tunnel to Demon World, and Shinobu Sensui would continue the work after his death, though for different reasons. Sensui wanted to die in Demon World so he could be reborn as a demon, seeing them as less evil than humans, while Sakyo just wanted to unleash the most powerful demons on the world to feed his own insatiable appetite for violence. He didn't care that other humans would be killed in the process, he only cared about the end result.
Sakyo was so evil and vile that his and the Black Black Club's torture and slaughter of lower-class demons caused Sensui to have a mental break and develop multiple personalities to cope with the trauma. Yet, despite his indifference to the lives of others, in the anime, he forms a relationship with Kuwabara's sister Shizuru after saving her from a group of demons ready to kill her. While the extent of the relationship isn't really explored as it's an anime exclusive, it's interesting to see a softer side of probably the most dangerous human on Earth at the time.
