I think, as it's been said, that your "evil-ness" is fine, bit psychotic, but hell, it's fun that way. The question you SEEM to pose is wether or not your chaotic. In my mind, the difference between chaotic and lawfull is simply your desire to uphold honor, or to stand beside your word. The nuetral ground, of course, is always a fun topic for discussion. I would say (with the exception of the details that obviously show you need counselling
) That you played it well. You attempted to get what you could, but got nothing. So your "chaotic" side (though arguably non-existant) pushed you to do what you felt like doing. And your "lawfull side (also arguably non-existant) had no vow or reason to uphold any shred of honor, as none had been posed. So, as the nuetral side was more lenient (sp), the evil won out and came forth stronger, giving you the desire to be evil, and basically dispose of the pirate, by whatever means you find suitable.
I guess my idea of the situation boils down to this
Nuetral alignment is forgiving in both directions of chaotic, and lawfull, wichever seems to suit, therefor is less likely to play the dominant roll in the characters personality.
Evil is a straight forward, flat out, bad and set in stone trait, making it more likely to play the obviouse and decisive roll in the actions.
So...
Evil>Nuetral
Wich results in the evil action taking dominance over the nuetral.
Your action was obviously evil, and not bound by either chaotic or lawfull, thus allowing you to flex slightly or swat between the two, wichever would better suit the evil tendancies. Your actions were undeniably evil (congrats), so I would say you played it fine.
(Yea, I know, way too long winded and most likely disputable, but oh well)