Izuku Midoriya, or Deku, started the series with no Quirk. He was destined to be a regular person, despite his worship of the Number One Pro Hero and Symbol of Peace, All Might. However, he proved himself to All Might when he tried to save Bakugo despite his lack of power, so All Might bestowed on him the One for All Quirk.
Unfortunately, this Quirk has some serious drawbacks, and it causes issues for both the user and those around them. Furthermore, Midoriya's diligence that stems from his gratefulness for finally having the opportunity to be a hero makes him liable to be rash.
8 He Repeatedly Destroyed His Body When He Should Have Worked On Control
Despite Recovery Girl's constant warnings and commands, and despite the fact he has seen what happened to his role model, All Might, Deku consistently depends on the power of One for All without any regard for the consequences.
He works on control as the series progresses, but he has an obsession with crawling up in power rather than saving his energy when, as Jujutsu Kaisen's Kento Nanami would say, only moderate effort is required. His insistence on using the Quirk not only makes his body condition worse, but it also makes things worse for the people who have to run in and help when he over-exerts himself.
7 He Tried To Save His Friends When He Had No Quirk
Midoriya has a heroic personality. Unlike Katsuki Bakugo, he is concerned with helping others more than he is with gaining power, proving himself, or fighting others. However, such a personality can lead to decision-making that is less than optimal.
Fans see this first hand when the Quirkless Midoriya stepped in front of a sludge monster that had attacked Bakugo. Despite Bakugo's Quirk, he had trouble escaping. However, Midoriya jumping in with no plan and no cards up his sleeve put himself in danger and made stopping the monster harder. Thankfully, All Might arrived.
6 He Remained Obsessed With All Might And Heroes When He Knew He Had No Quirk
Sometimes, one can revere their heroes too much, and this is exactly what Midoriya does with All Might. All Might's exploits were the reason Midoriya always wanted to be a hero, despite his lack of Quirk. However, All Might's message of saving others and never giving up might not have been the best advice for Mirdoriya.
Staying obsessed with All Might and other Pro Heroes is ultimately what inspired Midoriya to intervene with the sludge monster, and if he had not been given the One for All Quirk, he would likely be dead by now.
5 He Did Not Leave Saving Bakugo To The Heroes
The class 1-A students, led by Deku, wanted to help Bakugo more than anything when he was kidnapped by the League of Villains. They had lost him in the forest before this because they decided to fight (after convincing the higher-ups to let them) instead of jetting off.
Luckily, they rectify this situation by helping Bakugo escape when All Might thwarts the League of Villain's plans. They blasted over the battle area and grabbed Bakugo, making a quick escape. While the decision to fight the League in the forest did make things worse, it was soon rectified. The consequences were monumental, though, as this battle forced All Might to retire, the battle finally destroying his body.
4 He Went Haywire During The Competition With Class 1-B
Part of the One for All Quirk is its storing of past wielder's Quirks. However, Midoriya had little to no knowledge of this when he used one of these hidden Quirks. It went haywire, putting some of his classmates in danger.
He was trying to secure a win for class 1-A while in a tournament against class 1-B, but in doing this, he threw a wrench in the entire endeavor. Multiple students, including Ochaco Uraraka (who was injured), had to jump in to get a handle on the destructive situation.
3 He Did Not Save Eri The First Time
Waiting to save Eri was a tough call that seemed to have no right answer, but in hindsight, it may have been advantageous for Deku and Lemillion (Mirio Togata) to have saved her the first time they met her. Overhaul's intimidation was too much.
Because they let her go, the Pro Heros had to make a plan, and this plan led to Mirio losing his Quirk thanks to the serum Overhaul made using Eri's Quirk and Sir Nighteye (All Might's wise former sidekick) losing his life.
2 He Gave Iida Space When He Needed Help
Normally, when someone is grieving, letting them take some time for themselves is a viable way to support them. Midoriya seemingly did everything right. He told Tenya Iida that if he needed someone, he was there. However, Iida was volatile after Stain attacked his brother, and what he needed more than anything was some intervention.
By leaving the door open but not intervening, Midoirya failed to see his friend putting himself into trouble. His handling of the situation made Iida's resolve to stop Stain worse.
1 He Relies On And Gives Other Heroes Way Too Much Credit
In giving other heroes, including the heroes-in-training who go to school with him, credit, Midoriya fails to give himself enough credit and puts those heroes under too much pressure. This not only caused All Might to have to jump in on occasions where he could have sat out, but it also caused Bakugo's egotism to get worse.
Midoriya does this because he respects Bakugo and All Might, but they cannot always live up to his expectations, and Bakugo is too volatile for Midoriya to keep feeding his ego.