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Some instances that I feel show how some messages MHA are detrimental, especially on how victims react to their abuser, can be gauged by responses that tend to be highly prevalent in the fandom.
(Definitely not every fan, but a great majority).
Endeavor is a great example. Whenever you post criticizing his approach to atonement (and ultimately criticizing Horikoshi’s writing), you get BOMBARDED by people either belittling you for not liking his character or essentially forcing you to like his character by frantically writing “at least he tried” arguments.
If I have the CHOICE whether to forgive his character or not, especially given he goes through an atonement arc and not a redemption arc, why is any form of criticism about his abusive behavior and essentially his abuse of power practically ignored by the story unacceptable?
The message was detrimental because people operate on the notion that for victims to be good people, they must forgive and even help their abusers. MHA presents people who choose not to forgive him as either a monster (Toya) or inconvenient (Natsuo). And if they are still unforgiving, they must admire the abuser for doing the bare minimum (taking responsibility; this is also about Natsuo).
Essentially, they are considered "imperfect victims" because they weren't merciful in their approach to their abuser.
The majority of the fandom tends to ignore the lack of actual consequences for Endeavor's actions because he vows to talk to Toya every day. Insisting that doing the bare minimum, which is recognizing his son's existence and suffering, became his "hell" is a wildly fucked up message, in my opinion.
It harps on the issue mentioned above that if a victim isn't receptive to forgiveness or doesn't act "demure," they are seen as an inconvenience—which is how the Todoroki family ultimately views Toya.
On a less critical note, I'll vent, so if you don't like this, just ignore it.
I'm so fucking tired of stories depicting imperfect victims as people who deserve death and torture. Plus, having to be on the brunt of so many people acting like you're morally fucked because you're not impressed with how a writer handled abuse. Horikoshi is not the first writer to try to atone a character who is an abuser (and he isn't the first to fail at that, either).
I'm not about to dick-ride every decision every author makes. Especially if the message convinces some audience members that victims are inherently broken if they can't bring themselves to forgive and/or admire someone who hurt them.