The following contains spoilers for One-Punch Man by One and Yusuke Murata available in English via Viz Media.
Anime and manga characters oftentimes fall within certain tropes, such as tsundere, yandere, deredere, and other stereotypical traits. A chuunibyou is one of those character tropes. It is a Japanese slang term that roughly translates to "Second-year middle school syndrome," wherein the characters or people described as such hold a firm belief of being special. More often than not, it is a derogatory term to label a person or character as cringe-worthy.
While some character tropes affect the real world as people emulate their characteristics, chuunibyou has long existed before becoming an anime and manga character trope. As the trope becomes more established, more modern characters exhibiting chuunibyou make their debut. Rikka Takanashi from Love, Chunibyo & Other Delusions! and Megumin from KonoSuba are a few popular examples of this. Interestingly, Garo from One-Punch Man is also the very definition of having chuunibyou.
Ever since he made his debut, Garo has made it clear that he wanted to become the symbol of fear. He always empathized with the villains as a kid, often wishing to see the superheroes' downfall. As he grew up, his dream didn't change. That's what pushed him to seek power and challenge strong opponents to refine his martial arts skills. He fought and even hospitalized numerous heroes during that process, earning him the moniker Hero Hunter. Interestingly, his method worked. He grew stronger the more he fought and the stronger the opponents he faced.
Despite his stint as a Hero Hunter, he remained likable to the fans, as his struggles in his pursuit of power were somewhat relatable. The fact that he never actually killed anybody, and being a secretly soft individual further helped increase his popularity. Albeit striving to be the worst villain, he also surprisingly had a knack for saving others. He even saved the young Tareo on multiple occasions, often putting himself in danger during the process. Even so, his desire to be the manifestation of fear remained. And it only grew worse since then.
When a God granted him divine powers, letting him transform into his Cosmic Fear Mode, an opportunity to realize his dream presented itself. And he grasped it. He basked in the terror his new power brought, taking delight in the terrified expressions others expressed. He even pushed away both his former master, who constantly attempted to straighten his ways, and the young Tareo, who had recently just managed to firm his resolve. To cast away his doubts, he convinced himself by stating that absolute evil does not require anybody to understand it -- a line that is cringe-worthy in most contexts.
Garo is one of the most interesting villains in One-Punch Man. However, his actions in the recent chapters have made it apparent that he has chuunibyou. He wholeheartedly believes that he is someone special, someone who shall become the symbol of fear -- an ambition he had since his younger years. In pursuit of said goal, he pushed away both Bang and Tareo, who both remained loyal to him no matter the circumstances. He is far too involved in his delusions that he even has to convince himself that what he is doing is right.