Complete Monster/JAGverse

Much like the CSI Verse, the JAGVerse/NSISVerse has seen it's fair share of monstrous villains.

JAG

 * Charles "Charlie" Lynch from season 4 is a remorseless psychopath who kills at the drop of a hat for any reason. He begins in "Nobody's Child" by stomping, beating, and strangling a five-year-old girl to death, solely because he feels she may have disrespected him. After the murder, Lynch opts to target her sister for fun. While initially stopped, he later returns in "Goodbyes" to kidnap said sister and hold her hostage to torment her mentally before he plans on murdering her as well; we are also told that he killed his Master Chief and two other women, stomping one to death.
 * Mustafa Atef a.k.a. Mohandesh, the in-universe number 3 in Al Qaeda was captured by U.S. Army Special Forces in Afghanistan and given the death penalty by a military tribunal. When questioned about his part in the 9/11 attack on the World Trade Center, he responded "Want to know what I say about September 11? Every single person who died that day got exactly what they deserved!"

NCIS

 * Ari. After, he later . He casually manipulates innocents (giving a doll with a guided missile tracking signal inside it to a little girl)/sacrifices his mooks (who, given their affiliations, aren't terribly sympathetic either) just so he can get close enough to the team to hurt them directly. It is worth noting that (as foreshadowed when Ducky wonders what could create such a person) Ari does have a fairly elaborate Freudian Excuse, if even falls under that category. There have been more than a few worse criminals on the show, but Ari is mainly set apart in how he got to the NCIS team itself.
 * Kyle Boone from the episode "Mind Games"; he kidnaps and tortures women (and cuts off their tongues)! Think that's bad enough? He also snaps pictures of their suffering and keeps them in a journal (though he said he doesn’t need that; they will always be in his memories). His first victim was his own mother!

NCIS: Los Angeles

 * 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-comand, 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 Chameleon" in Season 3. To elaborate, this is the guy who often runs several cartels from behind the scenes, and the closest thing to an MO that he has is that he acts as the driver. He gained a wound on his mouth via G. Callen, who at the time believed him to merely be the driver. He then proceeds to kill a lot of people by incinerating them alive inside their cars, often covering himself by using various dialects. Afterwards, he plays a dangerous game of "hide and seek" where he forces people to act as him to lure them to dead ends, which often results in their deaths in the process. Oh, and he also kidnapped and killed a Russian police officer so he could pose as him and infiltrate the NCIS headquarters to "aid" them in finding him. His final appearance was his most monstrous depiction: He arranged to have Agent Roarke join up with a family of illegal Arms Dealers (and arranged for a fake exchange with some dealers he hired without informing them of the packages) with the intention of setting him up to be caught and for NCIS to get involved. Turns out, the family did fall for the trick against his expectations, so he sent anonymous tips to both the dealers and the NCIS agents revealing that he was an agent, and that his cover was compromised, respectively. After the NCIS team killed most of the family at the area (and arresting the accountant) to rescue him, he then sniped Roarke across the jaw, forcing him into ER before he died from a heart attack, and he kills the Red Herring sniper member of the family in order to leave a message to Callen, and he later kills Agent Hunter via Car Bomb in front of NCIS before he lets himself get arrested. He also arranged for Hunter to leak a video to NCIS via password about what he wants, and even the password while also making it seem as though he didn't want her to get it in order to lure Callen's team into a death trap while Callen kept watch over him (which they only survived after Callen deduced his plan), and he forces them into a Sadistic Choice to release him in exchange for an American agent who was caught in Iran and had valuable information of security weaknesses in the United States, and killing his allies at some point earlier (and it is also strongly implied near the end that the Iranians were going to be informed by the Chameleon anyways about the security weaknesses even if the prisoner exchange went off without a hitch). Even when he died, he ended up having Cullen's life potentially stigmatized anyways via a breaking news report when Cullen decided midway through the prisoner exchange that he simply cannot let the Chameleon live after the atrocities he committed, even if it meant disobeying his superiors and getting himself arrested by the LAPD.
 * 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 s 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.
 * 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 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.
 * Tahir Khaled is a terrorist from Sudan who serves as Agent Sam Hana's Arch-Enemy. First appearing in Season 3, he brutally murders three CIA agents and tortures Sam for information. He later arranges his sister Jada to be killed, whom he also regularly abuses only to have a recently rescued Sam save her. Upset only because 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 families 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 a bargaining chip for his release. Despite being release Tahir kills her anyway via asphyxiation, then proceeds to steals her body and starts to desecrate it. When Tahir manages to lure Sam to him in a bus he attempts to detonate bomb he placed inside said bus intending to take Sam as well as everyone else in the bus with him to the grave. Though lacking in high body count, Tahir Khaled makes up for in his petty cruelty against Sam and his family and friends cementing him as Sam Hana's personal nightmare.

NCIS: New Orleans

 * Mr. Stone appears in the last episodes of Season 3 as Mayor Hamilton's Dragon as well as one of the main investors of the latter's project to turn the neighborhood of Clearwater into a naval shipyard, which they plan to do as soon as possible despite 2000 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 kill Congresswoman Michelle Angelou, against the project, when he himself failed to do it himself. At one point he sends his men to fight Dwayne Pride in his bar while he plants inside destroying Dwayne's bar, killing his own men, and nearly killing Dwayne himself before he was able turn the tables on Stone. After that skirmish Dwayne later finds out that Stone plans to release the Floodgates onto Clearwater to drown the 2000 people still living there killing them. Though nominally subservient Mayor Hamilton, Mr. Stone's lack of regard for his boss and human life in general, as well as the Mayor's well intention's for New Orleans in favor for Greed and self-interest cements Mr. Stone as a greater threat, a more formidable enemy and overall amongst the franchises most wicked.