Can you imagine Naruto Uzumaki as a villain? What about Shoto Todoroki or Edward Elric? These anime heroes take action for the sake of helping others and improving humanity, but their life circumstances could have easily led them down the path of evil.
Most of these damaged heroes have violent or depressing pasts that could have led to them to a dark place. Naruto Uzumaki was shunned by his village because he had a demon housed inside his body. While he responds to this by wanting to prove his worth to his ninja village, he could have easily done what Gaara did and started killing the people who treated him like a monster. Edward Elric was willing to mess with bringing the dead back to life, but if his life had taken a different turn, he might have continued his experiments for more nefarious purposes, like Orochimaru.
None of these anime characters became villains, but in an alternate universe, they easily could have been.
Killua Zoldyck comes from a family of assassins that train their children to be powerful fighters from an extremely early age. In addition to physical prowess, the children are expected to repress their own emotions and betray and even torment each other in pursuit of power. Without some serious intervention from outside sources, as well as hard internal work on his own part, Killua could have easily ended up acting in accordance with his family's twisted beliefs.
His friendship with Gon, and his family's harsh and unreasonable treatment of his sister, inspires Killua to put himself on the right path.
Shoto Todoroki was raised to be a professional hero by a man he finds impossible to admire. Enji Todoroki, also known by his pro-hero name Endeavor, used illegal breeding practices to create Shoto's quirk, Hot and Cold. He proceeded to raise Shoto in isolation while abusing all four of his children and his wife. Shoto grew up hating his father, and dedicated himself to becoming a hero on his own terms. However, he could have easily become so disgusted by the concept of heroism - which his abusive father embodied - and pursued a life of villainy.
Like many of the villains in My Hero Academia, he could have decided that he was better off trying to destroy society than preserve it.
Naruto Uzumaki ended up developing a strong desire to help others, but it's something of a miracle that he turned out that way. His parents both died the day he was born after sealing the powerful Nine-Tails beast into his body. The people of Konoha feared the Nine-Tails, and Naruto grew up orphaned and totally ostracized by his community. While Naruto chooses to dedicate his life to becoming the leader of his ninja village and earning their acceptance, it's not hard to imagine how he might have grown up to resent - or even seek vengeance against - the community that turned their back on him.
There are actually multiple parallels in the series itself that show how Naruto could have turned out. There's Gaara, who was so isolated by his village because of his tailed beast that his own father started trying to kill him, and he became bloodthirsty and homicidal. Meanwhile, Obito Uchiha had a similar personality to Naruto, but was manipulated by Madara into believing that humanity was so wretched that they needed to be done away with altogether. The idea that Naruto could have become a villain - but didn't - is actually one of the main themes of the show.
Hitoshi Shinso's quirk (i.e., ability), Brainwashing, allows him to control other people's actions. The people around him see his quirk as inherently villainous, and many malicious characters suggest that he use it to force himself on girls or commit robberies. Shinso wants to be a hero, but what if he weren't so committed to this lofty goal?
What if, rather than push against people's unfair expectations of him, he decided to lean into it and use his power for evil? While there's nothing inherently evil about his personality, he could easily be tempted by the possibilities of his power, especially if the League of Villains happened to get in touch with him before a more heroic recruiter did.