Harmon Rabb: Charlie, there's something I need to know. Why did you kill (five-year-old) Annie?
Charles "Charlie" Lynch: I told her not to get on that school bus.
Harm: She just wanted to be with kids her own age.
Lynch: She disrespected me.
Harm: And for that, you beat her to death?
Lynch: You have a problem with that?
Charles "Charlie" Lynch: I told her not to get on that school bus.
Harm: She just wanted to be with kids her own age.
Lynch: She disrespected me.
Harm: And for that, you beat her to death?
Lynch: You have a problem with that?
- Charles "Charlie" Lynch, from season 4's "Nobody's Child" and "Goodbyes", is a remorseless psychopath. He viciously beats and strangles five-year-old girl Annie Lewis to death, solely because he feels she may have disrespected him. After the murder, Lynch opts to target Annie's twin sister Dar-Lin for fun. He later kidnaps said sister and hold her hostage to torment her mentally before he plans on murdering her as well. In addition, he also kills his Master Chief, as well as two other women, one of whom—the aunt of Annie and Dar-Lin—he stomps to death just like he did Annie.
- Kyle Thomas Boone, from season 3's "Mind Games", was a Serial Killer responsible for over 20 deaths. When he was twelve years old, he murdered his mother, and later devoted himself to starting a career in killing women. He would torture them to death, carve a heart shape in their backs, and would often take their tongues as trophies; he claimed these tongues were his "favorite sexual organs". When he was finally sentenced to be executed by the electric chair, Boone attempted to stay his execution and formulated a plan that involved setting up his lawyer Adam O'Neill as a copycat killer; Adam then kidnapped Agent Cassidy and attempted to torture her to death, though he failed. Boone was also convinced to take the families of his victims to the sites where their loved ones were disposed of. He took them all the way to a Virginia farm, only to dampen their hopes by showing that there was nothing in the barn other than the severed tongues of his victims.
- James Landis, from season 3's "Ravenous", is a Park Ranger who is also a Serial Killer. His modus operandi involves killing his victims, then making it look like a park accident or like a bear ate them. Three of his victims, with similar appearances, were beaten 7-10 days before they were killed. He had already killed 26 people, and had a 27th victim tied up and beaten, but was stopped before he could kill her.
- Karen Bright, from season 4's "Smoked", appeared to be a grieving widow and loving mother, but is later revealed to be much more disturbed. In truth, she is a Serial Killer who has killed many women, usually who look like her, in very brutal fashions either by slashing their throats or stabbing them repeatedly in the chest. She would then take the toes of her victims before burying the victims in her backyard. She has done this to 19 women and shows no remorse, guilt, and mercy for doing so, even taking sexual pleasure from it. It is also revealed that she killed her husband once he discovered her murders, all while cutting off her own toes to make him look like the killer. Once she is caught and gives an utterly terrifying Death Glare toward NCIS Agent Leroy Jethro Gibbs and FBI Agent Tobias Fornell, they are left utterly stunned on how quickly she went from crying to glaring.
- Louis Wexler, from season 11's "Alleged", was a Commander in the United States Navy who used his high-ranking title to rape the female personnel under his command. He does so by accompanying them to a bar and buying them a round of drinks, then dosing the drinks with sedatives before dragging them to their respective homes and raping them while they were unconscious. He has done this to nearly a dozen women, and those are just the women that came fourth. When confronted with the overwhelming evidence against him, instead of taking responsibility for his actions, he tries to blame the victims, all while claiming to be the innocent one, much to the team's disgust.
- Daniel Budd, from the final two episodes of season 12 and the season 13 premiere, was the leader of the Calling, a terrorist group responsible for recruiting children through the Internet for the purpose of turning them into terrorists or suicide bombers. Those he turns to terrorism are sent to kill several naval agents, while those he turns into suicide bombers blow themselves up in highly-populated areas; he sent a young boy to blow himself up at Cairo, Egypt, killing dozens, because an NCIS agent was located there. When the adoptive parents of a young boy try to convince him to leave the Calling, he has them killed. When one child is arrested, Budd has him killed just so he won't say anything. He then uses an adopted child to try to kill Agent Leroy Jethro Gibbs, with his next plan to destroy Shanghai and make it look like the North Koreans. His reasons for doing all of this was to kick-start a war simply because he wanted to kill thousands.
- Adam Connors, from season 13's "Déjà Vu", was a former NSA analyst who, along with Agent Eleanor Bishop and FBI Agent Daisy Milner, took down a group of Human Traffickers named Concubine. Adam, however, became greedy and recreates Concubine by kidnapping young college girls before beating them and leaving them in horrific conditions, tied up and threatening their families. Adam has those who resist personally executed before chipping the girls with GPS tags and selling them to international clients as sex slaves. After Daisy comes close to the truth, Adam kills her and creates a fake trail of evidence to exonerate him before trying to gun down Bishop after she finds out the truth.
- Robert Hill, from season 16's "She", abducted the then-18-year-old and pregnant Morgan Burke, and made her his Sex Slave, beating and raping her for 10 years. He would also abuse the daughter Morgan gave birth to for 9 years, who it is also implied he raped . When Morgan's daughter escapes Hill's captivity, Hill tries to cover his tracks by killing his own brother when he learned about his crimes, then attempted to murder Morgan and NCIS agent Eleanor Bishop at a lake.
- Malcolm Tallridge and John Bordinay, from season 1's "Ambush", are the heads of a dangerous militia group called "Enhancing Law Enforcement", a group known for killing immigrants or anyone who remotely looks like one, and a group which has stolen several anti-tank missiles. Tallridge, the leader, would order the hits, while Bordinay, described by Special Agent Mike Renko as a "paranoid nut job", served as his second-in-command, and would carry them out without the slightest hesitation. Their actions resulted in the deaths of over a dozen immigrants, including three children, who were chased into the desert and died of thirst. They are also just as brutal and ruthless to their men; when a member, Scott Reilly, brings in NCIS agent Callen into their fold without their knowledge, they have Reilly killed without a second thought.
- "The Phantom", real name Tom Smith, was a notorious Serial Killer who appeared in season 1's "Random On Purpose". Taking great pleasure in killing throughout his life, he started killing animals as a child, before moving on to humans as an adult. He traveled to many small cities where he would use his job as a receptionist to lure anyone, regardless of age or gender, into traps and kill them in various fashions. His killings grabbed the attention of Abigail "Abby" Sciuto, who travels to Los Angeles when she heard of his latest kill taking place there. When he notices Abby, he kidnaps her where he tell her about all of his murders and how she and him have a "connection" and Abby, in disgust, responds by calling him a creep. Feeling insulted, Smith then tries to decapitate her with an axe before being killed by the Los Angeles team who came to rescue Abby.
- Tahir Khaled, Agent Sam Hanna's Arch-Enemy, is a Sudanese terrorist and war criminal—demonstrated by a mass grave of his victims in his first appearance. First appearing in season 3, he brutally murders three CIA agents and tortures Agent Sam Hanna. He later arranges for his abused sister Jada to be killed, only to have a recently-rescued Sam save her. Upset that Jada will testify against him, Khaled sought vengeance against Sam during season 7 by targeting the latter's loved ones, ranging from launching a missile at his family's home, to appearing at his son's school with the intent to kill him, while callously killing innocents along the way. Upon being arrested, he arranges for his escape near the end of season 8 by kidnapping Sam's wife, Michelle, to use as a bargaining chip for his release. Despite being released, Tahir has her asphyxiated, then steals her body to desecrate it. When Tahir manages to lure Sam to him in a bus, he attempts to detonate a bomb he placed inside said bus, intending to take Sam as well as everyone else in the bus with him to the grave.
- Marcel Janvier, aka "The Chameleon", runs several cartels from behind the scenes, while he acts as the driver. He kills people by incinerating them in their cars and plays a dangerous game of "hide and seek" where he forces people to act as him to lure them to dead ends, killing them in the process. When infiltrating the NCIS, he murders and assumes the identity of a police officer to infiltrate the operation to capture him. In his proper debut, Janvier set up a family of arms dealers to be killed by the police and car bombs Agent Lauren Hunter before blackmailing the NCIS by threatening to inform Iran of a US spy working in Tehran. He later returns, with a hand severed, at the end of season 4, having been brought in by NCIS to broker a deal that would help them capture his old friend Isaak Sidorov, and offer him revenge against the Iranians for removing his hand. However, in the process, Janvier tells Sidorov that Sam is an undercover agent, resulting in the torture of Sam and Deeks. When Agent G. Callen confronts him about it, all the Chameleon has to say is that his goal in life is to now make Callen suffer.
- Peter Clairmont, from season 3's two-parter "Blye, K.", was a US sniper who killed a man in a drunken rage. To cover up his crime, he would murder over 15 potential witnesses, one of whom was his former comrade Donald Blye, who he beat to death, before making his death look like a car accident. When he learns that Donald's daughter, NCIS Agent Kensi Blye, was investigating her father's death, he would go on to murder the rest of his former unit, only leaving Alex Harris alive so he can bring Kensi to him, killing him afterwards. After engaging a brutal fight, Alex attempts to shoot Kensi In the Back, before he himself is killed by Owen Granger.
- Pritam Mondal, from season 7's "Seventh Child", runs a terrorist organization that kidnaps children in third world countries and raises them to become suicide bombers against the US, by brainwashing them into believing that their parents abandoned them and that the only way to reach heaven was to kill themselves in the name of Islam. He has done this to many children and some end up so psychologically damaged that they would rather blow themselves up than receive help. When one of the children rebels against Pritam, he still straps the bomb vest on him and his twin brother and forces them to blow themselves up in a public, with the twin brother getting killed. When NCIS breaches his headquarters he was more than willing to surrender to them just so he won't get killed. When NCIS agent Kensi Blye calls him out on his willingness to save himself despite telling children they should sacrifice themselves, Pritam simply responds with a grin.
- Joram Naser, from season 10's "Better Angels", is a Syrian general responsible for unleashing chemical weapons on his country's civilians, killing hundreds of innocent people, children included. When an NCIS agent was sent to investigate Naser, he proceeds to kidnap the agent before torturing him to death, constantly subjecting him to electrocution. When a Syrian doctor, David Sarraf, steals a flash drive containing Naser's atrocities, the latter rams David with a vehicle before leaving him to suffer, mockingly telling him that a good man would put a bullet in his head, to put him out of his misery.
- Otto Schmidt, from season 11's "The Circle", runs a human trafficking ring kidnapping immigrants throughout Europe and other parts of the globe to sell them off as sex slaves to the highest bidder. Schmidt is also willing to kill any of his prisoners when they try to escape, as seen when he personally executes one for fleeing, and when his human auction gets compromised in Los Angeles, Schmidt orders his men to kill the rest of his prisoners by burning them alive.
- Operatives "Lin" and "Davis", from season 3's "Quid Pro Quo", are agents from the Chinese government tasked in finding a subject to test a super virus the government recently created, though they go way beyond what was assigned. Upon arriving in New Orleans they kidnap three American citizens, then proceeded to burn them alive, two so they could take their identities and the third simply because they could. They then storm into a local bakery and force the baker to poison a cake with their virus, only to kill him with the virus once they are done with him. They ship the cake to a Navy base, killing seven Navy personnel, and later infecting Medical Examiner Loretta Wade with the virus. Despite already testing their virus, they decide to unleash their virus onto the air and into the city, attempting to kill hundreds if not thousands. This fails, and the team attempts to capture them alive due to their knowledge of where they are keeping the antidote. This results in one operative dying attempting to kill them, and the other committing suicide just so they could not find the antidote.
- Mr. John Stone appears in the last episodes of season 3 working for Mayor Douglas Hamilton, as well as one of the main investors of the latter's project to turn the neighborhood of Clearwater into a naval shipyard. They plan to do this by opening the floodgates, despite 2,000 people still living there. Under Hamilton's employment, Mr. Stone has already murdered nearly a dozen people, including a Marine investigating the project's shadier dealings, an innocent bystander to distract the team chasing him, and has even convinced Hamilton to kill Congresswoman Michelle Angelou, an opponent of the project, when he himself failed. At one point he sends his men to fight Dwayne Pride in his bar while he plants explosives inside, destroying Dwayne's bar, killing his own men, and nearly killing Dwayne himself before he was able to turn the tables on Stone. Dwayne later finds out that Stone plans to drown the 2,000 people still living there. Though nominally subservient to Mayor Hamilton, Mr. Stone's lack of regard for his boss and human life in general, as well as the Mayor's good intentions for New Orleans in favor of Greed and self-interest, cements Mr. Stone as a greater threat, a more formidable enemy, and among the franchise's most wicked.
- US Assistant Attorney General Eric Barlow, from season 4's last three episodes, got to his high position by framing dozens of politicians with crimes, while acting as the hero "uncovering" them. He intends to increase his power by orchestrating a terrorist attack during the New Orleans Tricentennial Fleet Week, hoping to kill thousands of attendees there, including New Orleans Mayor, Zahra Taylor, while replacing her with his puppet Deputy Chief Gosset. Barlow also makes sure no one interferes with this plan as he has Gosset and Amelia kill anyone who knows of it or his corruption. He also arranges many assassination attempt in the guise of riots against Dwayne Pride, and when they all fail, he decides to use Pride as his scapegoat for the terrorist attack, while he sweeps in after the attack to arrest him, pretending to be the hero.
- Eddie Barrett, from season 6, is a cult leader who recruits people by giving them drugs before brainwashing them into becoming his fanatical followers; among them Christopher LaSalle's brother Cade whom Eddie has murdered; Barrett later murders LaSalle while also trying to kill another of his followers to tie up loose ends. When NCIS and the FBI attempt to raid Eddie's, compound Eddie sets it to detonate intending to kill everyone within the blast radius, including all his followers, and orders them to shoot all the children being handed to the FBI. When his attempt to blow up his compound fails he abandons his followers to escape the authorities himself before goading Dwayne Pride to kill him by threatening to kill everyone Dwayne cares about.
- Larry Banks, aka Coach Eric Blair, from season 3's "Na Ki'i" ("Dolls"), was a perverted pornographer who drugged and forced women into taking part in sexually exploitative pictures or films against their will before he escaped to Hawaii and became a roller derby coach. Starting his enterprise up again and forcing his team to take part, he eventually drugs and offs a member who finds out the truth before later breaking the wrist and trying to murder another team member when he suspects she might be onto him.
- Ray Beckett, from season 3's "Ho'opio" ("To Take Captive"), runs a kidnapping ring where he has kidnapped numerous children whom he would abuse until they reach adulthood, where he proceeds to kill them by burying them alive. Amanda Rollins was only 7 years old when she was kidnapped, and killed ten years later. He would also beat his wife regularly to keep her quiet and cooperative. When the Hawaii Five-0 task force arrests him and demand why he did all this, he responds that he could profit from the welfare checks that were given to the children, and mocks the parents' grievance over the children that he killed.
- Ian Wright, from season 4's "Akanahe" ("Reluctant Partners") and the season finale "O ka Pili'Ohana ka'Oi" ("Family Comes First"), is a hacker who would steal security codes, then sell them to criminals who would use them to cause the deaths of thousands of innocents. Motivated only by greed and his own ego, he would later endanger over 300 people by threatening to cause a plane to crash if he were not released. His demands are met, but he still tries to crash the plane anyway. Later reappearing, he kidnaps Grover's daughter and puts her in a cage, in an attempt to force him to steal 100 million dollars for him and also murders an innocent old man just to get his house. He would later reveal that he never intended to actually let Grover's daughter go and was planning on killing her anyway even if he did get the money. Ian is a cold-blooded sociopath who cares only about money, and ranks among the very worst the franchise has to offer.
- Gary Nathan, from season 4's "Pukana" ("Keepsake"), is a cold, cruel Serial Killer who kidnaps young women and then has them tortured until they break, before dealing a gunshot wound to the back of the head when they're fully broken, having done this to 14 women. Gary, to relive the experience, removes one of their fingers and takes photos of their corpses before dumping their bodies. When a burglar discovers his finger collection, Gary kills him and puts Hawaii Five-0 on his tail as a result. Capturing Agent Chin Ho Kelly when he investigates, Gary begins to beat him and tries to break his will by showing him pictures of his victims before trying to murder his own girlfriend on the off chance she may find out what he's been doing.
- Jonathan Redmond, from the season 5 premiere "A'ohe Kahi e Pe'e Ai" ("Nowhere to Hide"), hacks into a military drone and decides to see if it works by killing two innocent hikers. Then, he uses the to terrorize the entire island killing as many people as he can, killing and wounding dozens, if not more. It reached the point where the authorities had to put the entire island on lockdown just so they could find without him killing any more. When the Hawaii Five-0 task force tries to find him, they learn that he was hired by a fugitive wanted for espionage to cause a distraction while the fugitive, who didn't want a massacre, would discreetly escape the island. Once they manage to track Redmond they asked him why he did all those things, Redmond responds that he would make money from it, and that this incident was far from his first.
- Nashid El Hamadi, from season 7's "No ke ali'i wahine a me ka 'aina" ("For Queen and Country"), is a terrorist infamous for bombing the British Embassy in Madrid, killing and injuring numerous people. He tries to top himself by attempting to uses a cyber key called Greystone to hack nuclear power plants all over Europe and destroy them, which would lead to a disaster ten times worse that Chernobyl and the death of thousands, if not millions. After his plans were foiled by the Hawaii Five-0 task force and MI-6 agent Harry Langford, who then proceed to arrest El Hamadi, he attempts to kill them by trying to shoot them In the Back.
- Madison Gray, from season 7, is a psychiatrist who manipulates her patients into becoming serial killers; she had one patient kill 8 people while having another kill 4 tourists by gouging their eyes out. She also kills a cop and staging his death as a suicide, while murdering another person before dumping his body on someone's bed. When Steve McGarrett and FBI profiler Alicia Brown investigate her, Gray stabs them in her home before subduing them and torments Alicia about her daughter's death. She then tries to have a murderous couple kill them while she leaves Oahu, going to California to corrupt someone into copying the Zodiac Killer's murders whom MacGyver would encounter. Returning to Oahu, she would reveal to Alicia that her daughter is alive and takes Alicia to her in Wisconsin, and threatens to kill her daughter, forcing Alicia to kill Gray.
- Emilio and Deon, from season 7's "Puka 'ana" ("Exodus") and the season finale "Ua Mau Ke Ea O Ka Aina I Ka Pono" ("The Life of the Land is Perpetuated in Righteousness"), run a human trafficking ring on Oahu where they kidnapped numerous young girls, some being as young as seven, and force them to become sex slaves, having their clients brutalize and rape them; some of these girls get shipped to the mainland. When Emilio gets captured and sells out his partner Deon for a reduced prison sentence, Deon takes the rest of his slaves to a truck, hoping to outrun the police chasing him, uncaring about the people he'll run over or if the girls might die.