It's sounds to me like Chaotic neutral
Chaotic: he is willing to do anything by any means to archives the results; that for me is like anarchy in most of his time.
Neutral: the form in which you describe him mean that he is not going to be a good boy or saint, at the same time he is doing something good in his eyes by eliminate this creature so in part he is not evil in my eyes.
As for the satisfaction when Killing this type of creature i see it like a drawback trait hedonism and the respect of a moral code like the honor sistem presented in the ultimate campaign
Chaotic Neutral at best. I'd even hazard he qualifies for Neutral Evil or Chaotic Evil depending how how willing he is to sacrifice innocent life.
Reasoning: Even Neutral characters have compunctions against hurting the innocents, though they are not completely unwilling to do so if it comes down to a 'them vs me' situation. If your character has no compunctions against doing so, then that's the difference needed to qualify for evil.
I agree with Deathless here. From the description, in regards to law and chaos, he's either Neutral or Chaotic, probably bordering on chaotic.
in the good vs evil spectrum, willingness to harm others just due to association borders on evil. So I would probably say Chaotic Neutral at best, Neutral Evil or Chaotic Evil depending on the killing of unwilling participants.
Neutral Evil.