A curse user is an evil human sorcerer who uses jujutsu with malintent. Naturally, they have existed as long as jujutsu sorcerers have, and were also prominent threats during the Heian Era alongside cursed spirits.
With the increasing number of curses in the modern age, being a professional jujutsu sorcerer is a demanding job, and not all those born with the talent for jujutsu use it to exorcise curses. Jujutsu High is always understaffed with jujutsu sorcerers and those they do have are always busy.
Before 1989, curse users could freely make money by operating in the underworld and doing any dark deed asked of them. Bounties are commonplace in the jujutsu underworld. These evil humans were able to live selfishly to their heart's content until Satoru Gojo was born. The threat of the Six Eyes was enough for a bounty of over a hundred thousand to be placed on the child's head. However, no one was ever able to claim it, and Satoru's overwhelming presence altered the balance of the jujutsu world. He stripped curse users of their freedom, keeping all evil forces in check.
Curse users remain active even in the 21st century but they are not as nearly as open about their activities. Organized groups such as Q and the non-curse users of the Time Vessel Association act mostly in secret. The Star Religious Group hired Toji Zenin to assassinate the Star Plasma Vessel while masquerading themselves as a religious order. Even the famous Sorcerer Killer was hesitant to confront Gojo directly. He used a dark website for curse users to enlist any curse user desperate enough to cash in on the bounty.
The Time Vessel Association believed operating through non-curse users would make them untouchable by Jujutsu High. However, once their plans were exposed, the organization was unable to talk its way out of the situation and was forced to disband..
Suguru Geto, a former student of Tokyo Jujutsu High, was expelled from the school after murdering over a hundred civilians. In accordance with article nine of jujutsu regulations, he was branded a curse user. Within the next decade, Suguru took over the remains of the Time Vessel Association and established his own group of followers from which he could collect curses and other resources. By 2017 he was recognized as the worst curse user of all.
Suguru aligned himself with several curse user commanders he considered his family. In a major curse user terrorist incident known as the Night Parade of a Hundred Demons, Suguru's group attempted to introduce an era where non-sorcerers would be exterminated. They ultimately failed and the remnants of that group continue to realize Suguru's dream by aligning themselves with Mahito and the other highly intelligent cursed spirits.
Along with Suguru's old commanders, several other independent curse users joined Mahito's faction in order to cooperate with a plan to seal Satoru Gojo. This would allow curse users to move freely again as well, and it would theoretically plunge the world into a state that's unfavorable for non-sorcerers.