I don't know what to call this kind of thinking from "One," but it's something close to nihilism.
For a long time, the culture has focused on "what is strength" and "how to survive by being stronger/more skilled."
In fact, yes, if a person wants to be the last person on earth, they will kill everyone and survive. The cost of that? They will be alone.
Apparently, the entire "militia culture" is kind of based on this premise: "survive by yourself and no one else." "One" took this theory and put it to the test, ignoring that in a militia, you have to leave a minimum margin of trust in a partner. "But I didn't like the person," so you walk away and that's it. It's said that you don't trust psychopaths that much.
Something that Ijin/001 understood in the end in a different way, especially because his surroundings helped him. In a war, even if you don't trust anyone, you should at least be grateful if they leave you some space to breathe, so to speak. Ijin's luck is that they took pity on him, and he can repay them by being a better leader than "One," even if it meant leaving the team.