Like heroes in many other Shonen manga, Black Clover's protagonist Asta makes a habit of turning defeated enemies and villains into friends, often regardless of their feelings on the matter. While Dragon Ball may be the series most notorious for turning enemies into friends, even it drew the line at genocidal maniac Frieza, proving that some villains are just irredeemable. Even characters like Piccolo and Vegeta had some atoning to do before they could be fully integrated into the group.
Where Black Clover stands apart, however, is that defeat doesn't equal friendship so much as it does amnesia, and the heroes are shockingly quick to forget just what crimes a character was up to before losing. With the wide variety of bizarre powers on display in Black Clover, some villains argue that they just had no choice but to be villainous due to their magic—something that goes unaddressed when they inevitably get pressed into helping the Black Bulls. It's basically a character trait of Asta's at this point to accept any overtures of friendship, regardless of past behavior or even the goals or intentions of the (former?) villain. While Asta's not supposed to be an intelligent protagonist, one would think his teammates might have some objections now and then. The "reformed" villains listed below are among the most egregious cases, earning forgiveness when they really should be regarded as beyond redemption.
Sally
Sally is a "magic researcher," using certain, very generous definitions of research. She's a very curious person, and fits pretty neatly into the mad scientist archetype, dedicating her time to experiments without regard for ethical implications. As such, she quickly left the magic knights when it became apparent they wouldn't let her do whatever she wanted, and ended up joining the Eye of the Midnight Sun because...well, they would. Sally's particularly fascinated by Asta's anti-magic, as well as some of the other rarer magic types exhibited by members of the Black Bulls.
Sally uses "gel magic," which manifests as a giant, gooey salamander that she uses to restrain potential test subjects. Her experimentation turns one henchman into a giant mud monster, and Sally tries to kidnap Asta and others at various points. She offers the results of her research to Licht and the Eye of the Midnight Sun, aiding them in their goal to wipe out humans and reinstate Elves, for which she's sacrificed as a result. Sally gets brought back to life to obtain vengeance on Licht, and ends up working with the Black Bulls when Asta volunteers to undergo whatever experiment she wants to do to him...which he had previously said he'd rather die than allow to happen. The Black Bulls could have avoided having Sally as an enemy altogether had he just said that in the first place! Still, with no regard for ethics and a penchant for human experimentation, Sally absolutely should not have been forgiven.
Rades Spirito
Rades Spirito is a necromancer who uses "Soul Corpse" magic, and a former member of the Purple Orcas magic knight squad. He's first seen attacking the city with his zombie minions out of a desire for vengeance over being expelled for his magic. Since people in Black Clover don't get to choose the magic they specialize in, banishing someone over it is wrong, but Rades' concerns are only for vengeance, and he too joins the Eye of the Midnight Sun. Rades and Sally work together a few times, and he's also sacrificed along with her. Fortunately for both of them, he's able to use his necromancy magic to bring them back to life, and they make their offer to work with the Black Bulls.
Rades' magic is creepy and inherently distasteful, and most who had it would avoid using it as much as possible. While he does have a valid reason to be upset, it really doesn't excuse the innocent people he attacks, and the trap he lays for Fuegoleon Vermilion nearly gets the captain killed. He's also vowed to kill Asta in particular, which brings him into conflict with Sally, since she can't very well study him if he's dead. While he's put to work helping to rebuild the Clover Kingdom, and eventually drafted into the invasion of the Spade Kingdom, he still hasn't even come close to making up for all he's done.
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Patolli
If the name Patolli doesn't ring a bell, it's probably because for most of his appearances, he was using the name "Licht." The real Licht shows up later, and doesn't approve of the things done in his name. Patolli was the leader of the Eye of the Midnight Sun and responsible for the plan to revive the Elves by taking over the bodies of humans, whose souls they reincarnated into. Both Sally and Rades were working for him, but he callously sacrificed them to demonstrate how little he cared for humans, even those on his side. Patolli had been a separate consciousness occupying the body of William Vangeance, using him to infiltrate the magic knights. While Patolli's desire for the destruction of humanity stems from the loss of the Elven people, he's completely wrong about how and why they died off, and has made himself a tool for the demon Zangred as a result. While he feigns empathy for his followers, it becomes obvious this is all an act when he starts sacrificing them as soon as they're no longer useful.
Once his plan is put into place, he steals the bodies of numerous magic knights and other characters, suppressing their human personalities in favor of those of the reincarnated Elves, who immediately begin rampaging. Patolli cuts off Fuegoleon's arm, attempts to kill both Yami and Asta, and nearly takes out the Wizard King himself. While Patolli eventually recants and volunteers to die willingly, he, too, gets brought back by Rades' magic, which places him into an artificial body, ostensibly to "pay for his sins." He's then allowed to go live in a peaceful village as its protector. Truly, this is a terrible punishment befitting attempted genocide.
The Witch Queen
While the previous three were affiliated, and eventually defeated, the Witch Queen is an odd one for Black Clover. As her name implies, she's the queen of the secret village of the witches, and mother to all witches who reside there--including the runaway Black Bull member Vanessa. She's absurdly powerful, and equally cruel and vicious. She should absolutely not be trusted under any circumstances, and it's only out of desperation that Vanessa turns to her, offering to return in exchange for healing Asta's cursed arms. Of course, that's not good enough for the Witch Queen, who instead tries to kill all of Vanessa's friend so she'll never have a reason to leave again. She also tries to force Asta to kill his friends himself using her blood magic to puppet his body, something only prevented thanks to Vanessa's fate-altering magic.
The Witch Queen is left in power after their encounter, and despite how dangerous she has proven to be to deal with, Vanessa consults her once again to ask for her help in growing stronger. While the situation with the Elves had forced her hand previously, there's absolutely no reason for the Witch Queen to work amicably with the Black Bulls, and she's arguably a bigger, more dangerous threat than any of the other kingdoms, if only because her forest is within the Clover Kingdom's borders. Filler episodes from the anime indicate that she's not particularly kind even to her own subjects, so if anyone in Black Clover deserves to be dethroned, it's the Witch Queen.