I don't think you can play NN as amoral and get away with it unless you've got a really niche role and some great RP chops.

I view NN as one of two things - either someone who really cares about the balance in CF (Nexun-y type) or one who really DOESN'T care.

If you're trying for the "doesn't care" outlook, a good example from literature might be Kitiara from the first three Dragonlance books (Dragons of XXX). She used to run with this group of do-gooders that included a paladin and a CG ranger type, but she got a better offer and became the head of the army of the evil badguys.

She's not crazed, and she doesn't change from kind to petty to flirty to dour...that's not NN, that's multiple personalities, and probably chaotic.

Her allegiances may sway, but her NN role is that she always looks out for number 1. She wouldn't kill some traveler alongside the road for the 20 copper coins he might have while she's wearing a full suit of armor, but if she were starving and penniless and dressed in rags? Yep he's a dead man.

Hope that helps.