- Season 1 has couple of them with two who are likely the worst; Kevin Carroll, who pretends to be a friend, but secretly murders poor Janet (The Hero's daughter's friend) in the most brutal way possible, and Andre Drazen, who, unlike his father (who barely avoids this trope due to love for his family), has no human feelings, and he brutally murders hostages, even after he gets what he wants.
- Surprisingly, none of the terrorists in Season 2 are Complete Monsters, at least not as much as Gary Matheson. Who is he? He is a father of the girl who Kim takas care of. What does he do? He constantly abuses her. While at first he may seem like more of a Jerkass, it is later revealed that he has beaten his own wife to death, and then he locked her body in the trunk of his car. He then blamed Kim for that and murdered the police officer. A true monster indeed.
- Recurring villain Nina Meyers is among the worst. While terrorists have the excuse of being motivated by ideology, Nina is driven by her bottomless greed. Having infiltrated CTU, Nina fed information to the Drazen terrorists to assist in their schemes, and when Teri Bauer overhears Nina speaking German, Nina murders her to keep her from connecting her to Germany. Nina assists in numerous innocent deaths and terrorist activities, even conspiring to allow terrorists to get their hands on nerve gas. Jack summed it up best: "You're worse than a traitor, Nina. You don't even have an ideology. You don't believe in anything."
- No (dis)honorable mention for Season 3 terrorist Ramone Salazar? He shot and killed his own brother to advance a deal on acquiring the Cordelia virus for a huge payout. Granted, Hector was distressed that his girlfriend got killed minutes earlier in midst of the deal, but damn, that was COLD. Even Nina Myers seemed to acknowledge such, and if your evil ways get her attention, you know you're seriously bad.
- While Habib Marwan is the ruthless Big Bad of Season 4, he pales in comparison to his Dragon, Navi Araz. When you forcibly involve your teenage son in terrorist activities to the point of ordering his girlfriend's murder on the basis that she's seen him outside a building that will later play a very brief part in the plan, then order your son's murder based on him being unwilling to commit the previous murder, you're this.
- Abu Fayed of Season 6, who detonated a nuke in LA in addition to committing the brutal torture and murder of innocents. He is responsible for at least 13,000 deaths.
- A big mention must be made of Philip Bauer, Jack's father, a Corrupt Corporate Executive who helped mastermind a conspiracy that reached all the way to the President, and helped have a previous president assassinated for it, with multiple innocents dying thanks to his actions. To cover his tracks he murders his co-conspirator younger son and later sells out his country upon being discovered, annoyed at the lack of appreciation for his work. Philip's work nearly started World War 3 and he kidnapped his grandson, fully willing to kill him if the boy didn't conform to Philip's standards of Philip's legacy.
- Alan Wilson, the real Big Bad of Season 7, was the mastermind of an elaborate scheme to force the U.S. government to let Private Military Contractors take over the military by staging a terrorist attack with the biological weapon Jonas Hodges and Starkwood developed for his cabal. While Hodges indicated that he honestly believed that the country would be better off this way regardless of the deaths the plot caused, Wilson is stated in a deleted scene to have stock in over a dozen private military firms and from his emotionless demeanor seems to be motivated by nothing but simple greed. The season finale then reveals that he was also the TRUE mastermind of Season 5, the same plot Phillip Bauer and Charles Logan were in on. This guy is so bad that Tony Almedia became a Fallen Hero just to try to kill him. Season 8 reveals that Renee Walker almost killed him in her interrogation of Wilson alluded to in the Season 7 finale, but it's hard to feel any sympathy for him.