@Acolytus They are portraying the single action he is doing as heroic, which it is. That doesn't mean though they are trying to say that "This single action makes Rudeus a total hero and awesome and better than everyone and above reproach." The manga itself drew our attention to the other slaves, deliberately, and gave us some close up shots. And highlighted that the kids were in worse conditions that what we saw the adults it. They wanted us to know clearly that there were more slaves that Rudeus wasn't even glancing at. Or do you think you're some kinda psychic that can see tings in a manga that the author and artist didn't show you on purpose? You only know those slaves are there and suffering because the author wants you to know. He wants you to see Rudeus' hypocrisy. If he wanted us to think of Rudy as a hero, Rudy would've saved the girl from some bad owners and she'd have been the only slave they owned, or at least the only one we knew about.
And your second paragraph, you totally miss the point. No one is saying that Rudeus' situation was the same as the girl's. Like I said to the other guy. Rudeus is talking about -feeling-. How the girl felt about life and how he felt. It doesn't matter if Rudy's feelings are 'justified' or where they came from. He is merely saying his -feelings- were the same then as the girl's now. Not that their experiences are equal. Hell Rudy himself even says that he was a garbage human. So unless you're saying he thinks this girl is garbage too, then he isn't comparing their situations. Go reread the first chapter, he is not excusing himself in his previous life, he absolutely hates his previous self and makes no excuses for how he acted. And that is true now, he didn't say anything he did back then was good, he simply said that is did it, he stated a fact. He doesn't condone his behavior or feelings from back then.
And as for parallels, you'd be right, if he was talking to people. The only thing he said to other's is "I've seen it before" reflecting on his past self was to himself. He didn't need to differentiate things because he already knew what he was talking about cause he was talking to himself. Do you explain to yourself in your head the differences between two things every time you think "Well these things are similar"? Pretty sure you don't. And they didn't explain it to the readers, because they gave credit to the readers to remember the whole theme of this story "Redoing my life as a better person, because I was trash before". They thought you would remember Rudy hates his past life. And didn't think you'd take a logic leap into assuming they're trying to justify Rudy.
@Redice the script is there for a reason. If you're not going to read what the script actually says, and instead make up your own stuff, go write your own book.