Defector: Well, yeah (nervously laughs)
Zurg: Promising. Any other bad traits?
Defector: I'm vengeful, cruel, and vain. Oh, I like to make people cry.
Zurg: Sold! We'll start you in the Yes-man pool.
Bringing your enemies over to your side can be a powerful tool — it allows you to weaken your opponent and strengthen your forces at the same time. However, there are some problems associated with such a strategy. For one, it makes infiltrating your forces rather easy, since sneaking in a spy shouldn't be too hard, but even genuine defectors might hold views incompatible with your goals and methods. Done on a large scale, assimilating former enemies into your forces may render said forces uncoordinated and prone to inner conflicts due to the differences between its members and groups.
Villains don't care. They will hire anyone they can.
In many stories the factions presented as evil will be far happier to accept traitors and defectors into their ranks than other groups, sometimes to an unreasonable extent. They will gladly allow former enemies into their ranks regardless of how much trouble said enemies caused them or how dangerous it might be. Some factions will consists mostly, or even entirely out of people who fought for the opposing side at one point in time.
This can be justified in a number of ways. Some organizations might simply lack a way to recruit or train new combatants and need to rely on defectors to keep their ranks full. Some villains might also have some unusually good method of bringing people to their side and keeping them there, be it mundane or supernatural. Besides, if the villains are consciously evil, they might just be happy to corrupt good-aligned individuals for the sake of it.
- The Demon Clan, the main antagonists of The Seven Deadly Sins, welcomes any member of any fellow Clan who defects to their side due to being outcasts themselves and doing so out of sympathy. They also value combat prowess, so the stronger the defector, the more welcoming the Demon Clan will be of their new member.
- Wizard Vilgefortz, the Big Bad of The Witcher novels, is perfectly happy to accept new members into his faction, even if those members have goals largely incompatible with his own agenda. In Tower of Swallow, he recruits Stefan Skellen and Leo Bonhart into his ranks and treats them as full-fledged members from thereon. He also attempts to bring Geralt over to his side several times, though his honesty is rather questionable.
- Percy Jackson and the Olympians: Kronos' forces consist in large part of demigods who betrayed the Olympus in favor of serving the Titans. Luke Castellan himself was a hero before throwing his lot with the Lord of Time. Aside from that, Kronos is perfectly willing to ally with deities who decided to switch their allegiance to him, at one point even offering to spare Hades provided he swears an oath of fealty to him.
- Ultimate X-Men: In the first arc, Cyclops got tired with the slow pace of Xavier's methods, so he left the X-Men and joined the Brotherhood. He had some concerns about the unneeded slaughter of civilians, but stayed on board. But, when Magneto takes control of an army of Sentinels, and head to Washington DC to raze the city, that was too much and he returned to the X-Men... convincing Quicksilver to betray his father in the meantime.
- The Forces of Chaos in Warhammer 40,000 consist in huge part of former imperials who fell to Chaos for one reason or another. Chaos Space Marines, in particular, are almost exclusively made up of former champions of the Imperium who switched their allegiance during the Horus Heresy, or during the ten thousand years that followed. In this case, the Forces of Chaos specifically value treachery and other negative emotions, plus Chaos explicitly has The Dark Side Will Make You Forget as an effect.
- Dungeons & Dragons:
- In older editions where Paladins were required to be Lawful Good, if they turned evil they could become a Blackguard and gain powers by making a pact with a Demon or Devil.
- Some evil gods include betrayal as part of their domain, so pulling an epic double-cross on your friends is a fast way to gain their favor.
- In Werewolf: The Apocalypse, the Black Spiral Dancers invite Gaia Garou captives to enter the Black Spiral Labyrinth and join their tribe if they survive. The Black Spiral Dancers also actively woo Gaia Garou who have the potential to fall to the Wyrm, and accept Gaia Garou who have been ostracized by their tribes or are fleeing from the Hunt.
- Star Control: Ur-Quan Kzer-za practice this strategy in regards to other species of the galaxy. They are willing to accept any race as their battle thralls, including the ones that had to be conquered first. In the second game, it is possible to make that strategy backfire on the Ur-Quan by convincing the freshly enslaved Yehat to rebel.
- Senran Kagura: In the first game, Hibari from the good guys' group poses as a Fake Defector to Hebijo school's ninja group. Their leader, Homura, easily accepts her because she just likes making new friends; that, and Hebijo's motto is "evil accepts all".
- Vampire: The Masquerade Bloodlines: Subverted if the player character joins the antagonistic Kuei-Jin. They direct the PC to defeat a mutual enemy, then decide that the PC is better off Rewarded as a Traitor Deserves and deal them a Fate Worse than Death.
- The Citadel DLC for Mass Effect 3 has the villain hiring a mercenary group calling themselves CAT-6; this is short for Category-6, the military code for dishonorable discharges, and is so named because said discharges make up the entirety of their forces. They're about as cohesive a force as you'd expect from a group of military traitors and washouts, and the villain only hires them for use as disposable cannon fodder.
- The Linear Guild, a group of minor villains in The Order of the Stick is constantly looking for new members to resupply their ranks and aren't above recruiting people from the opposing sides of the conflict. In their first appearance, the leader of the group attempts to recruit his good-aligned brother, an offer which gets swiftly refused. Later they make a similar offer to a fallen paladin Miko, which is then met with a rather violent refusal. Finally they attempt to recruit vampirised Durkon after killing his master, to which he responds by turning on them immediately.
- In She-Ra and the Princesses of Power, the Horde welcomes in one of the princesses with open arms. After Gadgeteer Genius Entrapta is mistakenly thought dead by the Rebellion, Entrapta becomes a member of the Horde, and Hordak welcomes her in. Even so, Entrapta joins less because she feels betrayed and more because the Horde has better access to First Ones tech.
- Played for Laughs in Buzz Lightyear of Star Command where Emperor Zurg takes over a planet Roswell and happily accepts the traitorous resident who offered allegiance as an evil henchman after he played up his negative traits to impress Zurg.