Note: Correct translation is as follows,
“Nami: ……Luffy!! Why?! No matter what his reasons, after he did all that to you…”
This confirms her reasoning behind the slap to Sanji and is coherent with my explanation above. I’ve said it before but I’ll restate it again. This is why I think that this moment is considered HUGE. Sure he could have made Nami say things from the top of King Baum but Oda also wants readers to get the message. Basically if you cross the line with Luffy, Nami will cut ties with you holy shit.
And to keep it short, I’ll only add in one more instance, her stand with Luffy against a freaking YONKO ARMY. As if a commander wasn’t enough, she stayed with her captain again, instead of leaving for safety, or again going after Sanji and the others.
She herself knows that this may be a losing fight. In fact she warned Luffy multiple times that they are not in the best spot, deep in yonko territory, with BM being angered.