Fallout 2: Frank Horrigan is a Secret Service agent and The Dragon to President Dick Richardson. A psychopath even before his mutation, Horrigan delighted in killing whenever he could. His first onscreen appearance has him gunning down a family, including their child, for refusing his demands. Later, he gleefully kills Brotherhood of Steel agent, Matt, and wipes out the entire community of peaceful, talking deathclaws that the player had earlier befriended. Horrigan has a special hatred for“mutants”, humans and creatures that have been exposed to the FEV virus or even the slightest amount of radiation. This racism leads to him aiding in the Enclave’s plan to unleash a biological weapon which will wipe out all mutated life in North America, in order to make America "genetically pure." Once defeated, Horrigan tries to activate the Enclave Oil Rig's self-destruct sequence, condemning all of his remaining allies to death, just so he can take the Chosen One with him when he dies.
Roy Phillips, a ghoul with severe hatred towards humans. If the Lone Wanderer negotiates with Tenpenny and his tenants to let the ghouls move in, eventually upon returning to Tenpenny Tower, all the human residents have vanished. If you check the basement, you'll find the corpses of the missing people stripped of all their possessions. Even Herbert Dashwood, who supported ghoul equality, is among the dead. If confronted, Roy tells you to fuck off before he adds you to the pile. Should Burke still be alive and nuke Megaton, Phillips will be ecstatic seeing “that smoothskin shithole” wiped out and offer Burke employment.
Dr. Stanislaus Braun, the Overseer of Vault 112 who controls the virtual reality simulation where he and the other vault members reside. A sadist with a god complex, for the past 200 years he’s been entertaining himself by torturing and killing every person inside the simulation, only to bring them back to life to torment and kill them over and over again. He specifically mentions that doing so is only fun because they are real people, and not computer simulations. Before Braun allows the Lone Wanderer to leave the simulation, he forces the player to do increasingly horrible things to the other Vault residents, culminating in the deaths of all inhabitants by the player’s hands. Braun was also the Director of the "Societal Preservation Program", better known as the Vault experiments. Advertised as providing civilians shelter from the expected nuclear war, in actuality the purpose of the Vaults was to gather unknowing, human guinea pigs to conduct horrific experiments on For Science! These experiments include forcibly turning Vault citizens into Super Mutants, tricking people into ritualistically sacrificing their fellow citizens, pumping hallucinogetic drugs into their ventilation system, and driving people insane by implanting posthypnotic combat suggestions into their minds through white-noise. Most of the Vault experiments resulted in the deaths, or worse, of everyone involved with them.
Vulpes Inculta, leader of the Legion's Frumentarii spy network. He engages in shady tactics that even Legate Lanius considers highly dishonorable. At one time when the Courier meets him, he razed Nipton to the ground* although the town was inhabited by Asshole Victims, enslaving, executing and crucifying most of the inhabitants while coldly imploring the Courier to examine it in detail before informing the NCR of his presence. He also coordinated a nuclear attack on Camp Searchlight, leaving it desolate and turning many of the surviving inhabitants into feral ghouls, and he was secretly arranging with the Omerta bosses to chlorine bomb New Vegas.
Cook-Cook. The Fiends themselves are always chaotic psychos who torture people for fun, but Cook-Cook is so bad that he is barely tolerated even by the other Fiends. He alternates between burning people alive and raping them, such as he did with Corporal Betsy and Pretty Sarah, the latter of whom he disfigured via burning. Two Westsider slavers, who only stopped raping their slaves because it lowered their value, were disgusted when Cook-Cook didn't even wait for them to turn around before enjoying a family he bought from them, starting with their son. One time, he bought some teenage slaves, 3 girls and a boy. The boy was burned alive and the girls made to watch. He is also implied to be a cannibal.
Big Sal and Nero, the Omerta family bosses. The only rule the Omertas have is "don't go against the family", which is punishable by death, but rather than emphasize loyalty, it is instead intended to keep others from standing in the way of the bosses' plans. Such plans involve secretly working for Caesar's Legion to massacre the entire population in New Vegas as soon as the battle at Hoover Dam begins, with a promise that they'll have control over the city. They are selling out the Mojave to the Legion not because they agree with Caesar's ideals, but they lust for power and loathe Mr. House for what they perceive to be his lording over them. They also get their prostitutes addicted to drugs to force them to be dependent upon their bosses, framed a man for murder to blackmail him into working for them, and give their hookers to Clanden to be raped, tortured and killed.
Clanden, an operative working with the Omertas. At first he seems like a perfectly normal, friendly man, but investigation reveals him to be a Serial Killer who makes snuff films of himself raping and torturing women to death. In exchange for a constant supply of hookers to kill, he makes chlorine bombs for the Omertas to use in their attack on New Vegas. If the player blackmails him into leaving New Vegas he’ll be okay with it, saying he can always “find more girls to play with” elsewhere.
Father Elijah, former elder of the Mojave Brotherhood of Steel and antagonist of Dead Money. His obsession with old world technology led to him getting his people slaughtered at the battle for HELIOS. After fleeing the battle, he sought out the fabled Sierra Madre Casino, a treasure-trove of lost technology which he wanted to use to "wipe the slate clean." In order to obtain the casino's treasures, he kidnapped dozens of people, attached slave collars to their necks, and blew their heads off unless they acted as cannon fodder for the Sierra Madre’s defenses. Once the Player Character reaches the Sierra Madre, Elijah orders him/her to kill off the allies that had helped them, and later tries to kill the Courier him/herself for outliving their usefulness. Elijah’s ultimate goal is to use the Sierra Madre’s technology to kill off every person in the Mojave, and then build a new nation where every person will be collared and forced to obey him.