Complete Monster/Metroid

Considering Metroid is one of Nintendo's darker franchises Metroid has its fair share of vile villains. But the ones listed here outdo all the rest as being the worst of them all.

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* Ridley, especially if you count the manga adaptation, is the series version of evil incarnate (this entry assumes the manga is canon). His first encounter with Samus (in the manga) has him landing on Samus' home of K2-L, and telling his minions to "First enjoy the slaughter to the fullest!" Not long after killing most of the planet's workers, he tries to kill the three year old Samus, but her mother saves her. He later eats her parent's bodies after Rodney Aran destroys his ship and badly wounds the giant space dragon. This comes up later when Ridley uses this fact to send Samus into a Heroic BSOD. In the games he does quite a few horrible things such as preventing Samus from confronting the titular Metroid Prime in the game of the same name (which would theoretically doom all of Talon IV considering what happens in the sequels), annihilating the space academy and everyone in it just to take back the Baby Metroid that Samus had rescued and adopted in *Super Metroid*, sadistically taunting Samus with the prospect of his return after his supposed Final Death to once again send Samus into Heroic BSOD before being interrupted by Anthony in *Metroid Other M*, and being all too willing to get in Samus' way in general, even when it doesn't directly benefit him, purely For the Evulz. He also has a bit of a narcissistic streak to him as well as shown by the mechanical monstrosity built in his image in *Metroid Zero Mission*. Brutal and sadistic to no end, all without saying a word, Ridley stands as one of the cruelest and darkest villains Nintendo has ever given us.
* Mother Brain was an advanced A.I. who was initially created with peaceful intentions by the dying race known as Chozo, but grew a god complex from her role. In order to take the universe for herself, she trained an orphan named Samus Aran to become her own personal Tyke-Bomb and then allies herself with the vicious Space Pirates while letting them attack the Chozo planet, Zebes, as she felt that she already surpassed the Chozo in terms of intelligence. After Samus discovers Mother Brain's plans in horror, she attempts to manipulate her into joining her side, and when one of the Chozo rebelled against her, she uses the leader of the Space Pirates to kill him. After Samus returned to Planet Zebes during the "Zero Mission" incident, it was revealed that Mother Brain intends to use a dangerous substance known as Metroid and pit the Space Pirates and the Galactic Force members against each other in order to reshape the universe in her own vision.
* Dark Samus is the Arc Villain of the *Metroid Prime* trilogy, as a recurring boss in the second game and the Big Bad in the third. Starting off as the titular Metroid Prime in the first game, who is a Metroid vastly mutated by the substance Phazon, it is defeated by Samus and apparently killed. But it uses one of her suits and DNA to reconstruct itself in her form with sentience. Now known as Dark Samus, she travels to the planet Aether to consume the Phazon on the planet and physically stabilize herself. She raids a Space Pirate colony on the planet and kills those in her way, and when Samus arrives on the planet, she tries to have her killed at every given opportunity. Defeated and left for dead in a collapsing dimension, she still survives and reforms in a Space Pirate ship, killing a third of the crew and forcing the rest under her control. She raids the G.F.S. Valhalla, kills its entire crew, and steals Aurora Unit 313 to control on Phazon and the planet Phaaze. She infects three planets with Phazon and almost infects a fourth, only stopped by Samus and three other bounty hunters. Dark Samus takes the opportunity to plant Phazon seeds inside the four of them, leading to the corruption and deaths the other three. Samus herself almost succumbs to corruption while trying to defeat Dark Samus. Ax Crazy yet also The Chessmaster, Dark Samus was perhaps the most effective villain in any *Metroid* game.
* In the manga adaptation of *Metroid Prime 2*, "Boss", the unnamed Space Pirate leader, raids the space ship Crest and tries, alongside his crew, to hijack it, killing a civilian who happened to be there. When the Crest Captain begs "Boss" for mercy, the Space Pirate overhears a worried child calming his sister down, saying that help is on the way. "Boss" torments them as much as possible before trying to murder them, as well as everyone else on board the ship, laughing at the older child trying to protect his younger sibling. When Samus comes to the rescue, "Boss" holds one of the children and the Crest Captain hostage and when Samus aims at him, he throws the hostages, chasing after them with the intent to slice their heads off.
* Gorea, the Big Bad of *Metroid Prime: Hunters*, is a formless, ancient alien that millennia ago crashed on Alinos within the Alimbic Cluster. In the form of a perverse imitation of the Alimbics themselves, Gorea sets about massacring the entire race to drain their life energy and empower itself, scouring entire planets of life, turning the Alimbics' own weaponry against them, and devastating the Cluster. Fully intending on continuing its rampage to the rest of the galaxy for the sole sake of power, Gorea's actions wipe out the Alimbics before it's finally sealed away in the Oubliette. Craving freedom, Gorea initiates the entire plot of the game by tempting bounty hunters into the ruins of the Alimbic Cluster towards its seal—and, upon breaking out, drains the power out of all the other bounty hunters before turning its attention to Samus Aran.