While I do agree with everyone that seeing shitty things happen to a shitty person is satisfying, it’s still worth pointing out that Rei wasn’t exactly dealt a favourable hand. His father was a POS who completely destroyed his self-esteem, and he internalised a reality where he sees himself as incompetent.
That led him to idolise and worship “amazing people”—people he sees completely as a far-reaching entity. Instead of trying to reach that level, he convinced himself he never could, so he just wallowed in that self-loathing and accepted it. That kind of mindset made him easy to manipulate, and Clover took full advantage of his desire to feel “special” and “useful.”
None of that justifies what he did, though. Making Uichi’s life hell just to prop himself up is still on him.
If anything, it just makes the situation more frustrating. You can see how his warped self-perception affects everything—he hates himself so much that even when someone like Uichi shows genuine kindness, he twists it into something negative, like thinking he’s being used as a foil to be laughed at. And that same mindset is exactly what made Clover’s manipulation so effective in the first place. Again, I'm by no means defending Rei's actions. Just because you're life is shit doesn't mean you can ruin someone's else life I'm fully ware of that. But it does show how badly his view of himself and others is messed up, and how that spirals into everything else.
Thanks for the TL