Anime is home to many trends, conventions and traditions, from the "cute witch girl" type to shy library girls to the unofficial isekai genre. Another long-running tradition is giving a shonen protagonist potentially villainous powers, then challenging them to master this dark power before it masters them instead.
Giving a hero dark powers is a simple and solid way to mix up the "good vs evil" formula, but it isn't limited to just one or two particular shonen anime. Many stories past and present follow this trend, and it may tie into the increasing trend of "action horror" in shonen and worldly themes that bring exciting but gruesome temptation.
When Horror & Darkness Become The Key To Shonen
The classic "good vs evil" paradigm is time-tested, but it can become highly predictable and formulaic in a hurry. Therefore manga artists, famous authors, movie directors and more seek simple but effective ways to mix it up, like creating sympathetic villains who are evil only due to circumstances or desperation. Another method is to add villainous aspects to the hero, making them a rough antihero like Guts the mercenary in Berserk or granting them a monstrous power. This freshens up shonen's classic "good vs evil" formula, where it's really a clash of true evil vs dark powers being used for good. Then it becomes a matter of how and why these dark abilities are used. Not all heroes need to resemble angels.
When a shonen protagonist uses powers darker than the villain's own, it falls to the hero to wield them for the right reasons and not succumb to the darkness. This can add some horror elements to the story, as the hero's upstanding psychological profile is challenged and pressured by the horrible abilities they wield. They might be defeated internally before even facing the villain, adding yet another exciting and tense layer to the conflict. Seeing an upstanding hero have their mind twisted and broken by their own power certainly has an element of horror to it.
Dark Powers & Shonen Heroes' Temptation To Use Them
A shonen hero wielding dark powers, such as Naruto Uzumaki or Bleach's Ichigo Kurosaki, might have an internal struggle based on controlling them. Others may struggle not with the power itself, but their relationship to it. Ichigo and Naruto were never tempted to abuse their inner monsters (Hollow and Kurama respectively), but other shonen leads such as Attack on Titan's Eren Yeager or seinen leads like Tokyo Ghoul's Ken Kaneki face serious temptation. They have obtained dark powers and can't easily get rid of them, so why not put that power to use and start forcibly remaking the world in their own image?
Eren soon grew comfortable with his Attack Titan powers, seeking even more to become invincible and launch the Rumbling. Kaneki nearly lost himself as he obtained a kakuja kagune, even turning on his friend Hide at one point and becoming unrecognizable to others. These dark aspects challenge the protagonist not to master their powers, but to master themselves -- and not all heroes complete this challenge. Eren became a total villain when he fully embraced his monstrous inner powers, while Naruto's Gaara of the Sand used his inner tailed beast as a reason to slaughter everyone around him. He was hated and ignored by everyone else, so he received validation and proved his own existence by shedding blood nonstop. Fortunately, Naruto saved him from continuing down this dark path.
Thematically, this may represent how people in real life can be tempted to take questionable shortcuts or even go down a dark path to get what they want, especially when desperate. Societal norms and the law say "don't do bad things," but people are nearly always capable of such acts. Shonen heroes make this temptation to abuse power palpable, and in a way, everyone has an inner Hollow like Bleach or an ever-present Sukuna like Jujutsu Kaisen that we must work to overcome.
