I agree completely with everything katiebell posted.

Yes, Harry Potter is just a book. Yes, it does have more signifigance to many persons in this world. But try putting yourself in his shoes; if someone was or was about to attack someone you cared for dearly and would possibly kill them, would you not react with the first impulse that came to your mind, even if it was something so treacherous? Not even Harry's morals could have blocked his reaction so quickly.

Although he is Harry Potter and knows more than anyone the difference between right and wrong, he is only human. As far as we know, no consequences ever came of that act, but perhaps in later years Rowling will address it and there will be no more need for discussion.

Also, it was for the greater good of both wizards and muggles, alike. The world owes him their gratitude for what he went through, even if he did happen to kill in the process. Was that one soul supposed to stand for more than the hundreds that died at Death Eaters' hands?