Why did the narrator killed porphyria?

The narrator is, of course, unreliable.  He interprets the look on her face as meaning that she has finally given herself totally to him, and to preserve that moment, he freezes it by killing her.  His insistence that she felt no pain is the most obvious evidence of his unreliability.  The speaker is insane.  The poem is primarily about his insanity--dramatic monologues are always about characterizing the speaker.