exploring shōto's understanding of heroism (and why i find the idea of shōto being a vigilante probable and fitting to his character):
i have mentioned this a couple of times before that shōto disagrees with authority (in general, if i'm honest) and the pro hero association rules. when the chief of police visited them in the hospital after their encounter with stain, he argued with tsuragamae regarding the rules of being a hero.
shōto: are you saying that we should've followed the rules and watched people get killed?
tsuragamae: are you saying that it's okay to bend the rules as long as it turns out alright?
shōto: isn't it a hero's job to save people?
like what is mentioned in this reddit post (which i suggest giving a read), 'he also questions the restricted laws made for heroes who he believes should do everything to save others and would not hold back even with authority.' when class 1A met the league of villains for the first time, we saw how shōto deals with enemies, having them wrapped in ice, making them aware that their cells are slowly dying and then leaving them under excruciating pain. he also exhibits no limitation in using his quirk against villains as long as he defeats them, having no regard if they are gravely injured or close to dying, even though this situation is against the rules of the association as long as he's doing the right thing.
before the events of the summer festival, shōto's goal as a hero is to be the number one hero, that is, his father's desire for him, the notion that was embedded in him. in contrast to the goal he held unto after his encounter with izuku, which is being the hero that he wants to become, he follows his own beliefs and idea of what being a hero is. growing up, i believe that shōto despised what a pro hero is due to his father's nature. the public sees him as a reliable and respectable hero who earned him the title of being the number two hero. in contrast to what he and his siblings saw, enji being an abusive father that caused tōya's death and his mother's mental instability. this upbringing caused him to believe that not everyone in the heroes under the association are good people (we all know we have a traitor in sight). to be an unaffiliated hero is better than being one as long as they save people at the end of the day.
in my opinion, the events of the summer festival affected shōto's character, showing that he has been deeply influenced by izuku that he started following his approach in heroism. of course, this is not bad considering that izuku has helped shōto realize what he truly wants to become and showed him that he is not limited by his father. however, this also caused a drastic change in his character, although they have similarities regarding their morals and beliefs. seeing shōto become loyal to his class and ua is a little bit off-putting (?) to me since the hero academy teaches them the rules to abide by, the same rules that shōto despises to follow. izuku also has the goal of being the number one hero (just like katsuki), the similar ambition that shōto once had, but upon seeing his father achieved this, he no longer sees the point in having the same title.
shōto also possesses quirks that are very similar to his personality, and that is being unyielding. he's not the type of person that can be controlled, for he follows his own sets of rules and ideologies. his belief of being a hero keeps everyone around him safe despite the possible consequences against himself or the enemy, but never to the people he wants to protect. this includes his hatred against villains, even though he can't see them as a person. his quirks are known to be prone to violence and destruction, quirks that he's willing to use against his enemies even if this means death for them (he definitely doesn't care about their well-being). this is in no way vilifying his character. still, i feel like this is the most realistic approach i see, considering his nature and the events that took place when he enrolled in ua, which changed his perspective in being a hero.
with shōto being a vigilante, we will get to see him use his quirks without limit and explore the different sides of his character. yes, he is a 'good' person. however, he is a force to reckon with, and if threatened, he will is ruthless and unmerciful. being a pro hero also has to do with what the masses think of you as a hero, this aspect of the job is something that shōto doesn't really give any regards to. he doesn't care about what people think about him as long as he keeps them safe and alive. people may see him as cold and indifferent, but that is a contradiction to his character who is willing to sacrifice his own life for the sake of others.
as a vigilante, i don't think he would be against the pro heroes (knowing very well that the 1a would be pro heroes in the future). still, he will certainly finish the job for them (like killing a villain that caused multiple casualties instead of having them spend a sentence in prison). of course, some may see his actions as something an enemy would do, however, like i have mentioned, this is the most realistic approach i see for his character.