This is all speculation, but keep your EYE on them, I think there's something there and I think there will be more.
In this post we're gonna tease out the difference between "using people as disposable lighters" and "killing was survival." Existing at a time where they had to come up with acceptable losses in order to survive, but Second was able to still ground what he was doing in kindness to someone, in saving someone.
Meanwhile I think everything AFO is doing is grounded in emotionally protecting himself. Like it has not actually worked, I'm sure the state of his emotional literacy is a complete garbage fire, but it's like—
"Well, nothing but me or Yoichi can matter. Yoichi's opinions on how Yoichi matters are inconvenient and hard for me, so I'm going to have to keep believing he's just foolish and mistaken until eventually I have to lean into being and enjoying being a cartoon supervillain"
His admiration of supervillains started WAY young, so obviously some of it was there from the start, but idk, I'm willing to bet what drew him to them was that they're the characters in the story who always seem unaffected, even gleeful. Especially considering the time he grew up in.
And how IS it that that's something so far distant from "killing was survival"?
Well, it's what distinguishes heroism, what ACTUALLY distinguishes it: save him first.
Now yes I could make an entire separate post about everything Stain gets wrong (he wants to die, he believes villains should die just as he believes bad heroes should die because he does not believe in people's capacity to change once they are too far gone, he does not care about people like Himiko Toga), but this thing is correct and played straight. Tenya Iida rolls it straight into his personal philosophy, and he brings it back in the JT arc with his announcement to the hero course that my priority is rescue, clearly framed as a triumph.
What makes a hero? Heroes must ground their fight in saving. It has to be about saving.
And one day, things will reach Izuku Midoriya's era. Deku will see the necessity of the hero who [tries to save] everyone, and Uravity will connect the pieces on the fact that we all need to become that hero.
It wasn't possible in Second's era, so Second chose what he could.
I think Second was deeply deeply messed up about the things he had to do in order to keep himself and other people alive. But I also think he kept asking himself why he did it, and kept investing in other people. There is a reason he understands "those who run alongside you."
And when Second introduces the notion of "those who will match your pace and run along side you," in that moment he means those who will STOP you when you need stopping.
There's a reason that that core Second belief is associated with what Iida is as a friend and a rival: Iida's entire philosophical rivalry with Midoriya is grounded in "neither one of us can make all the judgment calls alone, neither of us can make all the rules."
Sorry to bring Iida into it now, but Second sort of necessitates that for this analysis to make sense: The way Iida begins to stand back up and take heroic action is the advice "be the hero [your brother] would want you to be."
Iida was doing something for Tensei IN NAME, but it was based on Iida's own restlessness and putting his own emotions at ease. It was obviously something that it would be completely batshit to suggest Tensei wanted. The second he's confronted with that thought, instead of stuffing it into a very very distant little compartmentalized corner, Tenya knows that.
(Unlike AFO, Tenya also chooses to stand back up and do better this time. He chooses to take the critique, to understand that he was wrong despite how ridiculously harsh on himself Tenya is when he's wrong, and to dedicate himself to figuring out how to do better.)
Similarly, AFO takes actions in Yoichi's name, but they are, at every turn, entirely to soothe himself. They are never interested in what Yoichi wants from him. Investing in other people's answers when everything is already so hard is mandatory, but is difficult, and AFO chose the route that Second chose to rise above.
AFO had the opportunity to choose that too. There were people who, if he chose to do better, would have done the difficult work of running alongside him. I'm sure there was a long period of time when Yoichi would have— we watch Yoichi try to stop him with words, try to get him to do anything better, to at least stop this.
AFO was reacting to what it was to exist during his horrible, tumultuous time. And so was Second, and that's why I think foiling them is going to wind up essential to dig at the root of what makes AFO evil.
(Apologies that the picture wound up at no point being referenced in this post. Just to keep from confusing anyone, I'm gonna come out with it: that picture at the top is me finding the best AFO and Second panel I could to make this post look nice.)