Under those social assumptions, where one group of superpowered people is using their powers/"quirks" in a way which they assert to be for the benefit of society as a whole, they are automatically the heroes. Another group opposes them, & are automatically the villains. A better show than MHA would pull on that string, let the protagonist get in deep with the "hero" faction & then lead him to question whether it is in fact what it claims to be; but no more than about 20% of shonen anime, I'd say, have that level of self-awareness.
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Under those social assumptions, where one group of superpowered people is using their powers/"quirks" in a way which they assert to be for the benefit of society as a whole, they are automatically the heroes. Another group opposes them, & are automatically the villains. A better show than MHA would pull on that string, let the protagonist get in deep with the "hero" faction & then lead him to question whether it is in fact what it claims to be; but no more than about 20% of shonen anime, I'd say, have that level of self-awareness.