Season 6 of My Hero Academia depicts an all-out war between Pro Heroes and the massive Paranormal Liberation Front, with the heroes suffering more than ever. Several were killed in action, while others, such as Hawks and Best Jeanist, sustained permanent injuries. Then there's the protagonist, Izuku Midoriya, who has taken a significant psychological toll.
Midoriya has scars from his previous experiences as a budding hero, and he now has complete control of One For All. However, his mental state is deteriorating due to the strain, as evidenced by the rise of "Dark Deku," as well as his lack of a heroic smile like All Might's. Even the protagonist's costume, which has turned feral, is a troubling indicator of how much the normally upbeat Deku is suffering.
When Midoriya of My Hero Academia Acts on Instinct
A Pro Hero's costume and Quirk say a lot about their combat style and personality in My Hero Academia, and that includes wear and tear. Hawks is giving it his all to defeat the villains, hence his damaged wings, and even Endeavor now has scars. But Midoriya's wounds go beyond the ones on his arms; he is under serious distress and might not be thinking entirely rationally as a result. He is pushing himself too far to defeat all the villains himself, and he can't keep it up. The normally team-oriented, analytical Midoriya is now a feral beast, single-mindedly chasing after the strongest villains -- and his costume reflects that.
Deku thinks he knows what he's doing, but his attitude and ragged hero costume tell a different story. Not only does he have intense eyes under that hood, but his costume's mouthguard has become rough and jagged, like a set of beastly teeth. Similarly, his white gloves now end in sharp points like claws, and his costume's many rough edges and fabrics might remind fans of an animal's fur. Lastly, his costume's ear-like appendages -- which were once meant to emulate All Might's upright hair tufts -- really do look like an animal's ears or perhaps horns.
Everyone knows that as One For All's wielder, Midoriya is destined to fight and defeat Tomura Shigaraki so he can save the world, but the young hero's feral side is making him too single-minded about this goal. Deku is acting like a wild animal, recklessly chasing after villains by pure instinct and seemingly abandoning all rationality in the process. Now it's up to his classmates to "tame" him and snap him out of it. If they succeed, then Midoriya can send a better message by fixing his costume before the next outing or even swapping it for a fresh new look.
My Hero Academia's lead always looked strange with his original costume's hood and its ear-like appendages, and appears smarter and better with his head exposed. With Midoriya's current getup making him look like a savage beast without a human face, it's clearly time to switch back to something simpler and hood-free, showing everyone his smiling face as the new symbol of peace.
Deku, Like other Shonen Heroes, Must Control his Ferocious Instincts
A variety of anime characters use animal instincts and fighting styles, but a key factor sets them apart from Dark Deku in MHA. His instincts are controlling him at the moment, making a self-destructive beast out of him. By contrast, other animal-themed anime heroes focus on teamwork and use those instincts as a guiding tool. Midoriya is working against his own best interests and making a mess of things because he's pressuring himself too much to defeat Shigaraki right away, going entirely by instinct. He's even pushing away his teammates after being a great team player for so long.
This compares poorly to Demon Slayer's Inosuke Hashibira, for example, who actually wears a boar head as a helmet and uses Beast Breathing, having been raised by wild animals for most of his life. He looks even more beastly than Midoriya, but Inosuke smartly uses his instincts as mere tools, such as sensing creatures in a forest, and is a great team player. Likewise, Izuku Midoriya must get control of his more primal emotions in My Hero Academia; only then can he effectively use his more feral instincts with his rational human minds and defeat his enemies in a suitably heroic way.