Anime heroes and protagonists are usually underdogs who undergo exhausting training sequences to gain new powers, and sometimes, these heroes might even unlock or invent new powers while fighting the villain. These anime heroes are generally grateful for all the powers and abilities at their command, but a few others see things differently.
Certain anime heroes wield powers that come at a dreadful cost, or perhaps those powers/abilities remind the hero of a dark time in their life or some other problem. Whatever the cause, these anime heroes resent, fear, or even dread their own powers, and they won't use their abilities without good reason. Some heroes might even see themselves as monsters for having these twisted powers.
10 Edward Elric Realized What Alchemy Is Really Capable Of (Fullmetal Alchemist: Brotherhood)
The short state alchemist Edward Elric is usually comfortable using alchemy in battle and in everyday life, especially when he combines his alchemy with his expertise with science. There is an exception, though. Ed once attempted human transmutation with his brother, and he wouldn't dare do it again.
After learning some hard lessons, Edward now understands the true potential of alchemy. He suffered mental scars after he and Al lost body parts to create a zombie-like imitation of their deceased mother. That frightening experience ensures that Ed will only use honest alchemy from this point forward.
9 Mirajane Strauss Hated Her Powers At First (Fairy Tail)
By the main events of the Fairy Tail anime, the mighty S-ranked wizard Mirajane Strauss was no longer afraid of her demonic magic, but in flashbacks, she certainly was. In her youth, Mirajane despised and absolutely feared her Take-Over magic, which turned her into a battle demon. That's scary for any kid.
So, Elfman and Lisanna both learned Take-Over magic as well to make their eldest sister feel better about it, and that paid off. Now Mirajane is comfortable with her own magic, and she is more likely to make her opponents afraid instead.
8 Guts Is Wary Of His Berserker Armor (Berserk)
The dark seinen hero Guts can fight hordes of apostles with just his dragon slayer sword and the cannons and arrows built into his mechanical left arm, but once in a while, he needs more power. Partway through the Falcon of the Millennium Empire arc, Guts found and started using the terrifying Berserker armor.
While wearing that armor, Guts is faster, stronger, and more vicious than any mortal human, which allows him to win fights he normally could not even survive. It's effective, but it also takes a toll on Guts' body, and he and his party members are wary of it. He must not let this thing consume him.
7 Light Yagami Is Concerned About Other Notebooks (Death Note)
Light Yagami might not fear his own notebook, but after a point, he realized that other parties also had Death Notes, and he was deeply concerned about that. He felt invincible when he alone used the notebook, and that changed when Rem, Misa, Higuchi, and even Mello started using them, too.
Light was tense around Rem, since that shinigami threatened to write his name down if Misa was ever harmed. Light also worried about the loose cannon second Kira having a notebook, and he really didn't like Mello the criminal having one. Even Light might feel fear sometimes, after all.
6 Shuhei Hisagi Made Fear His Code (Bleach)
Soul Reapers are expected to understand the inner workings of their zanpakuto and make good use of their shikai forms and even bankai, if applicable. Meanwhile, Lieutenant Shuhei Hisagi fears his shikai Kazeshini because he appreciates its sole purpose — to take lives. It's an executioner's tool, and nothing else.
Shuhei learned from Captain Kaname Tosen that a warrior should only wield a weapon if they fully understand, and thus fear, those weapons. If a warrior doesn't fear their own weapons, then the warrior is taking their weapons too lightly, and that will spell disaster.
5 Yuji Itadori Would Rather Not Call Upon Ryomen Sukuna (Jujutsu Kaisen)
The cheerful shonen hero Yuji Itadori is like the new Ichigo Kurosaki, a good-hearted punk who has an insidious villain on the inside. Instead of an inner Hollow, Yuji is the vessel of the mighty Ryomen Sukuna, the brutal king of curses. Yuji can call upon his power, but doing so is highly risky.
Yuji won't always admit it, but he's clearly terrified of Sukuna and won't interact with that curse king if he can help it. No one can blame Yuji for being afraid; the last time he called upon Sukuna, that curse king tore his heart out. It's a miracle Yuji came back from that.
4 Shoto Finds His Fire Powers Disturbing (My Hero Academia)
Some students in class 1-A are at least comfortable with their Quirks, and a few of them are very happy with the Quirks they were born with, such as Katsuki Bakugo and his explosive Quirk. Then there's Shoto Todoroki, the natural genius whose Quirk comes with serious emotional baggage.
Shoto fears the fire half of his Quirk and what it represents since it was inherited from his tyrannical father, Endeavor. Shoto mentally blocked it off and stuck to his ice, but during the UA sports festival, Izuku's words got through to him, and Shoto began to overcome his fear of his father and those flames.
3 Ken Kaneki Was Frightened Of His Ghoulish Nature (Tokyo Ghoul)
The hit seinen anime Tokyo Ghoul stars the bookish Ken Kaneki, a college student who didn't want anything to do with ghouls. Then, Rize Kamishiro attacked him, only to have her organs transplanted into him. Now Ken is a half-ghoul, and he was shocked and horrified when he realized that for himself.
Ken swore to suppress his ghoul side at all costs, terrified of his new power and all its implications. Over time, though, Ken overcame that fear and embraced his ghoulish side at last, and even reveled in it. The fear was long gone by then.
2 Naofumi Iwatani Is Careful With His Wrath Shield (The Rising Of The Shield Hero)
The bitter isekai hero Naofumi Iwatani eventually unlocked the powerful but scary Wrath/Rage Shield, which had a mind of its own and could possess Naofumi to some extent. Naofumi made use of it for a time, but he found it disturbing and soon decided to keep it locked away if at all possible.
Naofumi never really like the Wrath Shield, and Raphtalia found it especially terrifying. Later, Naofumi truly embraced the power of friendship, and the Wrath Shield could no longer be summoned on command. It's just as well.
1 Zenitsu Agatsuma Is Shocked By His Own Power (Demon Slayer)
Zenitsu Agatsuma is afraid of many things, mostly demons and the threat of death, and he is always reluctant to join a battle, even though he has a combat-ready Nichirn sword on hand. If he gets too frightened, he will pass out, and then his real power will awaken at last.
Zenitsu's Thunder Breathing involves ultra-rapid, deadly strikes in a straight line, and Zenitsu finds it terrifying. He will sometimes awaken soon after an attack and realize "Wait. It was me who did this?!" He simply doesn't like scary powers at all, not even his own, since he fears violence most of all. At least he's got some serious moves.