To be honest, I’m not sure if Vin’s right—if Kelsier should have stayed back from the trying to save the people—or not. It’s certainly the more heroic thing to try and save them. This scene is to show that Vin still has a little bit of her Reen-crafted selfishness (or, maybe self-preservation-ness) left in her. Kelsier is ready to risk everything for his friends. You can debate whether this impulse is foolish, but I think it’s noble.
Vin’s sin here isn’t deciding that going after them would be too dangerous. It’s how quickly she jumps to this decision, and how powerless she decides that she is. She’s not a coward, nor is she ungrateful. She’s just lived on the street too long. In a situation like this, her first instinct is not to fight, but to flee. (Just like it was when the army got attacked by the garrison a few chapters back.)
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