Stain. While he has valid points about the shortcomings of the current hero system, he ends up attacking or even murdering heroes who fall short of his standards, like when he ends the genuinely heroic Ingenium's career, and nearly kills Ingenium's younger brother (who, while blinded by revenge, came to his senses with Midoriya and Todoroki's help).
This sometimes happens to most of the members of the League of Villains, as fans focus on their tragic backstories and interesting personalities while ignoring the horrible crimes they have committed.
Tomura Shigaraki has his fair share of fangirls, especially after his woobie traits have been revealed. A lot of his fans went on to forgive his murderous rage against a bunch of fifteen year olds because of this.
The Cute and Psycho Himiko Toga is a sociopathic, blood-obsessed killer, but fans attempt to justify her behavior and ship her with the protagonist regardless. Never mind the fact that she has sexually violated three first-year students since then, and has been confirmed to be so madly in love with one of them so much that she transformed herself as her out of love.
Dabi has his fans if only due to Evil Is Cool and his interesting design. The theory that he's related to Todoroki and/or Midoriya has also helped somewhat (as many expected, he's Endeavor's son Toya). His fans somewhat expect him to get revenge against Endeavor and get away with it.
Bakugo gets this treatment from some fans, particularly those who ship him with other characters (Midoriya in particular, but to a lesser extent when he's shipped with others as well). Midoriya seems to be guilty of giving this treatment to Bakugo as well if you consider he always goes out of his way to excuse Bakugo's behaviour towards him. It's also common for some fanfics to portray his relationship with Midoriya, when not romantic, infinitely more amicable than what it really is, sometimes even as Vitriolic Best Buds, and both of them often recurring to each other as an advisor or confidant. A particularly odd little trend that's especially popular on Tumblr is claiming that Bakugo's anger at Midoriya is completely understandable, if not justified, as Bakugo spent years training with his Quirk and becoming stronger so he could reach the top only to be usurped and ultimately end up in the shadow of his rival who was "handed" a powerful quirk after having once been powerless. This completely ignores the fact that the story actually shows that it was the other way around (Bakugo was the one who was naturally gifted and mastered his quirk almost immediately while Deku had to train his body for months prior to obtaining OFA specifically to keep his body from being horribly mangled by it), Bakugo bullied Midoriya viciously before Midoriya inherited One For All, and also comes with the rather disturbing side-effect of making it sound as though the continued bullying afterward is Midoriya's fault.
Gentle and La Brava got this a lot. While both of their stories are tragic and sympathetic, people often ignore the fact that they are still criminals. Some even go as far as say that Midoriya is the one being the jerk in that situation for not letting Gentle go on with his dream and that he is the true hero there. However, Gentle's selfishness has also been pointed out, as he admits to be willing to destroy Eri's chance at learning how to smile, as well as all of the work that the heroes-in-training put into the U.A. Cultural Festival, simply just to get some sort of attention.
Endeavor. Since the start of his redemption arc, many fans have taken to glorifying him and generally demonizing all of his family, denying he was ever abusive to begin with by claiming it was just Tough Love. A downplayed variant is to acknowledge his abuse of Shoto but deny he mistreated his other children (especially Toya/Dabi for fans who dislike him) even though Parental Neglectis a form of abuse regardless of if Shoto got the worse end of the stick.
To a minor extent, but though he does care about them, Aizawa can be a real jerk to his students at times, but his more diehard fans tend to excuse every single thing he does/says towards them, even when he's being way harsher than he needs to or messing with them for his own amusement.
Even All For One gets this. While plenty are willing to admit that he is a Card-Carrying Villain through and through, some think that deep down, he actually cares for his brother and/or Tomura Shigaraki, portraying him as a protective brother/father figure. This blatantly ignores him imprisoning (and possibly even killing) his brother when he refused to join his side, and possessing Tomura Shigaraki's body while admitting to using him for his own ends. There are also some who see him as a Well-Intentioned Extremist who's genuinely trying to create a new and better world containing quirks, even though his brother calls him out on this being a lie just to hide his true power-hungry nature, which All For One doesn't really bother to deny. 407 even flat out confirms that All For One doesn't see his brother as anything but a possession. No doubt that him rewinding to his past form in the final battle furthered this.
