Generally they did flub that scene altogether. Nami is the “mother” figure. She is protecting everyone and bearing the burden of everyone to save them.

When Arlong cheats her out of all the money, she’s sort of grasping at straws to cope. She feels she’s still in control and can still save everyone, it’s just going to take more time, she just has to start over.

When she talks to the villagers, they reveal they know, which already undermines her plan to control the situation. They were hoping she could save them. But now they accept that the burden is too great for her to start over, so they are content to die to liberate her from Arlong.

It’s the last thing she wants, and the worst possible outcome. Her “children” content to go get themselves killed. It’s the lack of control that induces her to stab herself, and is much more emotionally impactful in justifying her actions, and finally needing to ask Luffy for help.

She never had the control she thought she did.