This is a really interesting sequence of panels, We have Spandam’s evil cackling while Robin is shown to be in a lot of emotional pain and turmoil. But the thing that caught my attention the most is Oda’s decision to focus on Luffy’s expression on the bottom panel.
Robin admitting that she wants to die marks the start of the big turning point in her arc. For the first time she’s being honest about her feelings instead of just trying to run away. And yet, for such a momentous admission, the focus is on Luffy’s surprise.
The obvious contrast is Robin’s later “I want to live” speech, which ends up being the most famous Robin panel in the entire series. I think what Oda’s emphasizing here is that Robin doesn’t want to die so much as she wants acceptance, and she thinks that she’ll never get it no matter who she meets or where she goes. Once she has that reassurance from the Straw Hats that they’re with her for the long haul she finally has that hope again and finds that will to live. In short, Robin saying she wants to die here is the start of her being honest, but we’re still not at the heart of the issue, so the focus stays with Luffy.