>>283708358
It's a matter of Denji's perception, not yours. When he does something that makes him feel bad about himself, like killing Aki, he immediately second guesses himself and wonders if he could have handled it in another way.
He's not Mr. Superman who has total moral clarity and knowledge of all the factors involved. He's a dumb, uneducated man with no parental guidance, trying to navigate through situations that he isn't accustomed to, because for most of his life, he only had to worry about himself.