The key is that once she got her town back she kept on stealing, and with no guilt about it. We're also not talking about petty change here. Lastly, what is she doing right now? We just got more evidence of her robbing people blind.
Sure, if you can tug on her heart strings, Nami will help you. That's really not a big deal. I encourage you to look at Rich's articles on gaming, in particular about how evil characters can still have friends, etc.
Actually, from the looks of things she stole something that they had multiples of, because its the one thing that might let her navigate what are supposed to be the most dangerous waters out there.
For her to be evil, we'd need to see her do something that either intentionally does harm, or does harm accidentally and she doesn't care.
Pilot of the Thing
I'm not evil. My morals just aren't the same as society's.
On a one man quest to beat the Star Wars Universe, using nothing but simple, plain, ordinary logic. Score so far: Me 593 SWU 450