In any anime series, the main protagonist is usually seen as the epitome of justice and the one who uses their powers to defeat the evil force. However, many of these protagonists have dark traits residing inside them and also face situations that they will not be able to avoid that can ultimately lead to their transformation into the villains that they despise.
While some are driven by the need for vengeance, others may want to test the limits of their abilities. Some of the anime heroes that fall under this category have what it takes to become an antagonist in another series according to their situations that they find themselves in.
10 Tamahome (Fushigi Yuugi)
Tamahome is introduced as one of the Seven Suzaku Celestial Warriors in the Fushigi Yuugi series. One of the strongest characters in the series, Tamahome is a confident and clever protagonist who is always willing to sacrifice himself to save Miaka. Tamahome has immense physical strength, and he is also an experienced swordsman. As a Suzaku Celestial Warrior, Tamahome has the ability to increase his battle skills and channel his chi into energy blasts.
While Tamahome is the epitome of an anime protagonist, his willingness to do anything for his loved ones may spark a dark change in him. In any other anime series, if his friends and family are harmed, directly or indirectly by the main protagonist, then Tamahome will not hesitate to go down the villainous path to avenge them.
9 Rem (Re:Zero)
Rem is one of the main characters of Re:Zero and a powerful Oni. During the series, Rem is also seen to have magical powers that she can use to attack and heal. Rem has a painful past as her parents were killed and her village destroyed by the Witch Cult. Driven by the force for revenge, Rem is not afraid to kill anyone whom she believes has a hand in the assassination of her parents.
Rem is an extremely clever character in the series, and she has no boundaries when it comes to avenging people she loves. Her ability to harness magic to kill someone makes her an exciting prospective villain for any other anime series.
8 Kai (Beyblade)
Even though Kai is one of the main characters of Beyblade, he has always maintained his cold demeanor. However, his willingness to sacrifice everything to get what he wants makes him a great character with the potential to become a memorable antagonist. Kai's mysterious nature and habit of maintaining his distance with others are also some of the biggest factors that can aid his transformation into a villainous character.
As a villain in any other series, Kai would be tactful and quick in spotting the weakness of his opponent. Kai's ability as a powerful Blader can also be added to the list of why he can transform into a very capable antagonist.
7 Saitama (One-Punch Man)
As the main protagonist of One-Punch Man, Saitama is the strongest character to exist in that series, or most series in fact. Since he can defeat his enemies with a single punch, Saitama suffers from a self-imposed existential crisis and wishes to recover from it. Over the One-Punch Man series, Saitama fights many characters but he is always able to defeat them with just one hit.
Saitama's full power is still a mystery. His ability to defeat any person makes him a formidable villain if Saitama ever wished to be one. Along with his physical abilities, Saitama is also tactful and clever, which is a bonus to his strengths.
6 Katsuki Bakugou (My Hero Academia)
Katsuki Bakugou is a rather unstable character in My Hero Academia. As one of the main protagonists, Bakugou has the required positive traits, but it often gets overshadowed by his aggressiveness and superiority complex. Bakugou's competitive drive is often an area of conflict between him and his classmates.
Bakugou's careless and often violent behavior makes him a possible villain in any other series. His fiery attitude along with his strength would allow him to be a fitting match to any of the protagonists of different anime series such as One-Punch Man.
5 Lelouch Vi Britannia (Code Geass)
As the main protagonist of Code Geass, Lelouch is a highly intelligent leader who leads a rebellion against his home country, Britannia. While he is compassionate to his loved ones, Lelouch is often known by others to be clever and selfish when it comes to winning over his enemies.
Along with his sharp mind, Lelouch is considered as one of the strongest characters of Code Geass because of his Geass that gives him the ability to gain a person's complete obedience. Lelouch's skillful strategizing combined with his insanely strong powers makes him a strong antagonist for other anime series such as Re:Zero.
4 Himura Kenshin (Rurouni Kenshin)
Himura Kenshin, also known as Battosai, is the titular character of the Rurouni Kenshin series. While Kenshin may seem easygoing and humble at the beginning, he holds many secrets related to his past that he wishes to forget. Kenshin is an accomplished swordsman and is considered one of the most powerful warriors in the series. Kenshin's various unique sword techniques make him a force to be reckoned with.
Kenshin's past as an assassin makes him an extremely strong villain in any other series. Especially if his loved ones are in trouble, Kenshin will not hesitate to raise his sword and shed some blood.
3 Sasuke Uchiha (Naruto)
Lonely and misunderstood, Sasuke Uchiha is one of the main characters of Naruto. Growing up with the rage of avenging his slaughtered clan, Sasuke focused only on training and being the best version of himself to be a strong ninja. However, along the way, Sasuke learns the truth about his parent's death and redeems himself with the help of his friends, Naruto and Sakura.
Sasuke's cold demeanor and the willingness to go to any lengths to achieve what he desires would make him an interesting villain in any other anime series. His strength as one of the strongest ninjas would also add to the advantage that he would need to win over the main protagonist.
2 Ken Kaneki (Tokyo Ghoul)
Ken Kaneki is introduced as the main protagonist of Tokyo Ghoul. Originally a normal human being, Kaneki's life is transformed when he receives organs from a ghoul that turn him into half-human, half-ghoul. While he initially has a hard time adjusting to his new life, Kaneki soon gains immense power and becomes one of the strongest ghouls to exist.
Since Kaneki has the ability to destroy anything that obstructs his way, he would be an ideal villain in any other series. His unbalanced temperament along with his strength would make him a perfect antagonist in series such as Parasyte.
1 Rikuo Nura (Nurarihyon No Mago)
Part-human, part-yokai, Rikuo Nura is the main protagonist of the Nurarihyon No Mago series, and has been raised to lead the Nura clan. While his human form portrays a weak and sincere boy, Nura's yokai form, which comes out during the night, is strong and confident. As a yokai, Rikuo is well-trained and can become merciless if his beliefs or his loved ones are provoked.
Rikuo Nura would be a perfect villain if circumstances leads him to be one. As an antagonist, Rikuo would shine as a villain in other anime series such as InuYasha.