Well, it really comes down to what you mean by ok.

Is it morally ok to kill a person? Personally, I don't think it's ever morally right to kill someone.

Is it ok legally (meaning you won't go to jail)? It depends. First, there aren't any enacted statutes or "laws" if you will, that say it's ok to kill a human being. "Unlawful" simply means without a defense or justification such as insanity or self-defense.