[Top Gun]

A few years back in school I read a short story where 2 worlds are at war. The author makes it sound like the character you're reading about is human, but at the end you find out he's actually at war with humanity. The last bit of the story he kills a person(described as creature?) said "it had four limbs, pale white skin and let out a scream when it was killed" or something like that.

Anyways the story was quite good and I'd like to find it, can anyone help?