These flashbacks show how people outside the Academy experience the great catastrophe, and with Robin... More than wanting to humanize it, it's like watching a bomb counting down, Parents always absent, no friends, he only has his sister, The great catastrophe, He tells him to take care of himself so that he doesn't get hurt, His sister dies because she didn't want him to yell at her again, that bomb exploded the day Haruki died, And I don't even want to imagine what happened between 2026 and 2033.
Very good points about Robin's upbringing. People who think Ishiguro would do a cliched "actually he was good all along!" cheap twist don't really understand his writing or ethics, but also expect some "pure cartoon villain right outta the womb" explanation too. Both approaches to writing antagonists are weak, but I think people have been fed so many bad "sad villain backstories to instantly redeem them" in shounens or whatever that they don't understand what's happening here, but it's very simple - it's to show how a human being has devolved into such a state to begin with, which is not a justification of their behavior, but rather helps underline their immorality more emphatically than any supernatural explanation ever could.
Remember the Hotel King arc, where Maru wonders, "If I'd have grown up here, would I have joined their gang and done bad things?", and Kiruko answers by warmly holding his hand -
"I don't know, but we have each other now, so that won't happen". How much are humans a victim of circumstances, of the manner of their births, from where you grew up, or from a series of bad mistakes that snowballs into each other? How close is anyone to being a bad person themselves if the same happens to them? Is redemption truly possible, like Toru, even after he murdered a child? Perhaps the answers aren't so black and white.
Also funny that Marin was somehow reading Crime and Punishment last chapter before she got the seizure.