Complete Monster/Metal Gear

Canon

 * Colonel Volgin from Metal Gear Solid 3 is a sadist who, within minutes of appearing, fires a Davy Crocket nuke at his countrymen just to give it "a test drive," knowing full well that he won't be blamed for it, even when one of his subordinates is clinging to his arm, begging him not to. Then come one of the most vicious Cold-Blooded Torture scenes in video games. Note that it's damn clear he didn't care if the torturee lives or dies before getting the proper information, as long as he gets his kicks. Then we also have the fact that Volgin, long before the events of the game, had participated in the Katyn Forest Massacre as a NKVD member, where he managed to kill over 20,000 Polish officers with his bare hands, and not only that, but he also removed their blindfolds before beating them to a pulp, and he killed several people in both the Hungarian Revolution and the East Germany Uprising, which he not only had absolutely no regrets about killing 100,000 anti-communist activists, but even boasted about the feat.
 * If you think about it, Volgin's "test-drive" (and probably Coldman's involvement in getting Volgin into doing his "test-drive")
 * Sundowner from Metal Gear Rising: Revengeance is a sadistic war profiteer who gleefully kickstarts wars for the hell of it, due to his beliefs that humanity's natural state is a violent, bloody one, and he shows this at the beginning of the game when he sadistically assassinates President N'mami (The president of a foreign African country). What really qualifies Sundowner for this status though is that the guy isn't afraid at all of exposing children to the horrors of war: he kidnaps orphans off the street and removes their brains in order to be put through intense war simulators. The kicker? The children are aware of this the entire time, and after all the brutal trainingis over, they're forced out into war to commit atrocities similar to the ones they'd commit in Sundowner's simulators. While even Armstrong has surprisingly altruistic reasons for going through with this plan, Sundowner is a sick bastard who gets his jollies from the suffering of humanity.

Non-Canon

 * Although not canon it has some pretty horrific people. Metal Gear Ac!d 2 has a Complete Monster, and surprise, it isn't Dr. Koppelthorn, since he did have a legitimate excuse for what he did (as his wife was killed and wanted to bring her back). No, the real Complete Monster was Wiseman. He instigated a horrific event known as the Praulia Massacre as the Model 2s did not provide satisfactory data in his eyes, and sent in the Model 3s to be cleaned up. He was also responsible for the death of Koppelthorne's wife, which also resulted in the events of what happened later on. Finally, after gaining the Lucindia File, he had Venus execute Snake simply because he outlived his usefulness. Speaking of Snake, he also had Venus render him amnesiac in the first place. Although he ultimately was arrested by the FBI for war crimes, his reaction in the ending suggests that he will end up escaping custody in some way or another.
 * Metal Gear (NES version) & Snake's Revenge: This non-canon version of Big Boss lacks all redeeming qualities of his canon counterpart. Leader of Outer Heaven, Big Boss kidnapped Dr. Drago Pettrovich Madnar and forced him to create Metal Gear, a bipedal Walking Tank capable of launching nuclear missiles from anywhere in the world, threatening to kill his daughter, Ellen Madnar, if he refused. Solid Snake managed to destroy his Metal Gear, free his prisoners, and blow up his base, leading to Big Boss barely surviving, but needing to replace his damaged body parts with cybernetics. Desiring revenge, Big Boss started to recreate Metal Gear, planning to use it to launch nuclear strikes across the whole world, with the first targets being New York, Tokyo and Moscow.

Metal Gear Solid (IDW)

 * Liquid Snake is one of the 3 clones of Big Boss, who was jealous of his status as the most inferior. Growing to hate Big Boss, Liquid became furious upon hearing that Solid Snake killed him; angry that he "stole his chance for revenge", Liquid organized a scheme to take over the nuclear base on Shadow Moses, using his FOXHOUND unit to get his hands on a new Metal Gear Rex, which he planned to use to force the US government to hand over the body of Big Boss, and when they refused, he has DARPA Chief Donald Anderson and ArmsTech President Kenneth Baker tortured for the codes to launch Metal Gear. When that failed, he manipulated Solid Snake into activating Metal Gear and tries to kill him with it, brutally crushing to death Solid Snake's ally Grey Fox, who he had previously brainwasher, in the process. Managing to survive by possessing the body of Revolver Ocelot, Liquid first managed to take over his body when he was about to steal Metal Gear RAY from USS Discovery tanker, and used this Metal Gear to tear this tanker apart, killing dozens of people who were onboard the tanker. Taking over Ocelot's body again, when Ocelot betrayed Solidus Snake and taken over Arsenal Gear, Liquid sent Arsenal Gear towards Manhattan, escaping before it crashed into the city, destroying a big part of it.
 * in the adaptation of Sons Of Liberty, Solidus Snake is the self-proclaimed best of the 3 clones of Big Boss and former President of the United States, known as George Sears, who was deeply involved in the First Liberian Civil War, personally going into the field and killing countless people, innocent and otherwise. Building his own army of Child Soldiers there, Solidus at one point killed the parents of Jack and adopted him, and makes another child soldier for his army from him by abusing him his entire life and forcing him to commit murders, like during his 10 year birthday he forces Jack to brutally kill 10 of his hostages as a "birthday gift". After Jack was taken away from him by the Patriots, Solidus, angry that the Patriots took away "a precious resource" from him, soon started to create a plan of overthrowing the Patriots and usurping their power. When the Patriots found out about this, Solidus escaped, bombing a Paris hotel as a cover up, framing Solid Snake in the process. Forming his own army "The Sons of Liberty" and hiring the Dead Cell unit, Solidus took over the Big Shell facility, planning to get the names of the Patriots from Arsenal Gear and when that's failed, he tried to get them by attempting to extract the brains of a grown-up Jack, who became known as Raiden. Scheming to abandon Dead Cell on Arsenal Gear, so they would distract the government from him, Solidus wanted to kill the Patriots and be hailed as a hero by the public for returning their freedom.
 * In the same story, Fatman, aka "The Emperor of Explosives", is one of the members of the Dead Cell terrorist group and the vilest of them all. Having a fascination with bombs, Fatman, as a child, creates a nuclear bomb, thus earning his nickname after the bomb dropped on Nagasaki, and while still a child he killed at least one of his classmates by blowing him up, all because he wanted to build himself a name and hated that classmate for daring to think himself smarter than Fatman. Helping Dead Cell commit their crimes by creating explosives and assisting them in taking over the Big Shell facility, Fatman became a spy for the Patriots, just for the chance of killing his former mentor Peter Stillman. Chaotically blowing up parts of the facility solely to kill Raiden, Fatman rigged the entire facility, desiring to destroy it and to kill all hostages and his own teammates, if either Raiden or Stillman would have bested him.