In the My Hero Academia universe, a hero's usefulness is judged based on their Quirk and how effective it is in hero work. The limitations placed on Quirks are to prevent the loss of life since some have more destructive abilities than others. Flashier Quirks tend to get all the shine, but there are many more understated Quirks that would be terrifying in the hands of a villain — that is, in the hands of someone with no limitations or morals.
While abilities like Yo Shindo's "Vibrate," Toru Hagakure's "Invisibility" and Thirteen's "Black Hole" are the obvious choices for villain-y Quirks, there are many others that get overlooked. With society prioritizing showier Quirks, the more minor or mundane ones aren't given much thought, yet could be the most dangerous in the wrong hands. Let's look at a few less obvious heroes and heroes-in-training with Quirks that would make them devastating villains.
Vantablack & "Black"
Shihai Kuroiro, hero name: Vantablack, is an aspiring hero in U.A. High School's Class 1-B. His Quirk "Black" allows his body to merge with anything that is dark in color and move freely within it. This extends to anything naturally or artificially dark as well as anything darkened by external factors. This means that Shihai can hide and move even through shadows, completely concealing him from his opponents and making him especially terrifying as a nighttime stealth operative.
His Quirk also grants him enhanced speed, which makes his stealth capabilities that much more deadly. While operating during the day poses a challenge, moving through the night presents endless opportunities for a quick and easy infiltration or kill. After all, in the dark, no one can see you coming.
Midnight & "Somnambulist"
Midnight is a Pro Hero and teaches modern hero art history at U.A.. Her Quirk, "Somnambulist," creates a sleep-inducing aroma from her skin that renders nearby opponents unconscious. It has a stronger effect on males than it does on females. While Midnight uses weapons to make up for her Quirk's lack of combative abilities, as a villain, Somnambulist could easily put her at the top of the food chain, taking combat out of the equation altogether.
During the final exam in the anime's second season, Hanta Sero was put to sleep via Midnight's Quirk and stayed asleep throughout the exam and thus, he failed. Though it can only be used in close range, getting hit with that aroma can mean game over and be an easy win for Midnight. In a more withdrawn, villainous role -- and given Midnight's own somewhat sadistic personality -- Somnambulist would make quick work of anyone infiltrating her lair, for example, as closed spaces are perfect for this kind of Quirk.
Ragdoll & "Search"
A Pro Hero and member of the Wild, Wild Pussycats, Ragdoll's Quirk "Search" grants her the ability to observe and monitor up to 100 people at a time, learning their weaknesses and location in the process. Although Search isn't a combative Quirk, it gives Ragdoll all the information a strategist or assassin could ever need. It's the perfect recon ability, one that gives her the upper hand in any situation and allows her to plan ahead of time by providing useful information to predict her opponents' movements.
As a villain, the large scale of her Quirk would also be handy in observing any heroes that might be on to her, letting her stay multiple steps ahead. Search could make any crafty bad guy nigh untouchable — there's a reason her Quirk was stolen by All for One in Season 3, there's just no beating the ability to spy on your enemies from anywhere.
Anima & "Anivoice"
Koji Koda is a student at U.A. and is in Class 1-A with a Quirk called "Anivoice" which allows him to conscript any animal into his service, big or small. While Koji is merely a side character in the series, there is no limit to what or how many animals he can command. He has been seen to use a flock of birds as a smokescreen and commanding an army of insects to swarm Present Mic.
As a villain, Koji could very well instruct vicious and carnivorous creatures to do his dirty work for him. He could easily cause a bear attack or bug infestation to come down on a whole city, leaving no evidence of his involvement if he plays his cards right since Anivoice can be used from long distances. With this Quirk, Koji would be a villain with practically untraceable, completely loyal minions at his constant disposal -- no one would suspect the movements of animals to be the work of a person, after all.
Mei Hatsume & "Zoom"
Mei is in U.A's support department Class 1-H. Her Quirk is "Zoom," which means she can zoom in on anything up to five kilometers away. While this Quirk doesn't seem like much, it would be useful for someone like a sniper, or really any kind of hired gun. This, combined with her skills in creating support equipment, could easily allow her to be just that.
If Mei were to use her equipment to attack from long distances with the help of her Quirk, she might be hard to catch. Attacks could come from so far away, there'd be no way to find the attacker. As a villain, there are no rules, and she could also become the largest distributor in illegal support equipment -- and their most effective user. What she creates can be outside the confines of the law, making the possibilities both endless and devastating.