Of the many characters in My Hero Academia, none stand out more as a potential bad guy than Katsuki Bakugo. Set up as the archetypal shonen rival to Izuku Midoriya, Bakugo epitomizes negative energy unlike any other, including actual villains in the anime. Therefore, it's unsurprising that many fans call him an anti-hero and have found ways to align him with the story's antagonists.
Bakugo takes his job as a shonen rival a little too seriously. Not only does he provide an ample amount of obstacles for Midoriya to overcome or challenge, but he always does so in the worst possible mood. Bakugo's mannerisms are terrible. He's constantly yelling, berating others, and doesn't care about anything that's going on unless it involves himself. In every external respect, Bakugo would make the perfect villain.
But what sets Bakugo apart from the villains of My Hero Academia is his utter belief in All Might, just like Midoriya. They just show their dedication in very different ways. Bakugo and Midoriya are two sides of the same coin -- both dedicated to All Might and his ideal while encompassing different aspects of their passion. Bakugo's unpleasant mannerisms often mar what are very good intentions, and only Midoriya can see that.
His love is entirely too strong to be corrupted, as seen when he was kidnapped by the League in an effort to break him down and turn him to their side. Though domineering and negative, Bakugo will not be so easily swayed by evil. He's worked and trained too hard to simply give up and become a villain. There would have to be a very good reason for him to change sides. And even then, Bakugo's stubbornness and pig-headedness will probably get in the way.
Bakugo becoming a villain would be entirely too predictable and would take away from what My Hero Academia is all about: the struggle against adversity in all its forms. For Bakugo to easily give in to his inclinations toward aggression would be lazy writing. Plus, we get enough of Midoriya vs. Bakugo to satisfy that craving for him as an antagonist.
It's also notable that his Quirk is particularly aggressive and would make sense in an evil, villainous scenario. But Bakugo trains hard to use every facet of his Quirk to stop villains, not help them. He takes his violent, explosive ability and turns it on itself, using it for peace instead of violence.
Bakugo's role is best-suited as the shonen rival he already is. He provides enough negative energy and antagonism for a villain, yet we see him fight with the good guys and (mostly) for the right reasons. To have him switch roles would be boring -- there are enough villains in the My Hero Academia universe already. While he's motivated by beating Midoriya, he has high expectations of himself and the hero he would like to become. Bakugo could never be a villain.
