She started out as a Yandere who pretended to love him to get revenge since he didn't save her father from a really messy death, then truly fell for him when he showed her genuine kindness... but she could only sort-of tell him her true feelings after she was murdered by the Big Bad.
After an initial reaction of genuine anger, followed by questioning Nojiko (the one person Nami told about her plan to free the village), they pretended to be ignorant of her motives and acted like they hated her so she wouldn't feel obligated to keep working for Arlong.