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.
Although the Metal Gear Ac!d series isn't canon to the main series, even 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.
Sundowner from Metal Gear Rising: Revengeance is a Blood Knight war profiteer who seeks to spread War for Fun and Profit around the world, believing it to be the natural state of mankind. He begins the game by killing the President N'mami of the country Raiden begins in whilst being needlessly sadistic, and has street children kidnapped while their brains are removed into cyborg casings while they are aware the whole time. We see hundreds of these brains as well. They re forced to commit endless atrocities in virtual reality before they are sent off to commit them in the real world as well.